Diet and lifestyle are believed to play a large role in whether colorectal polyps form. Studies show there to be a protective link between consumption of cooked green vegetables, brown rice, legumes, and dried fruit and decreased incidence of colorectal polyps.

Because of the way familial polyposis develops, it is possible to have the genetic condition, and therefore be at risk, but have no polyps or issues so far. Therefore, an individual may be diagnosed "at risk of" FAP, and require routine monitoring, but not (yet) actually have FAP (i.e., carries a defective gene but as yet appears not to have any actual medical issue as a result of this). Clinical management can cover several areas:

- Identifying those individuals who could be at risk of FAP: usually from family medical history or genetic testing

- Diagnosis (confirming whether they have FAP)—this can be done either by genetic testing, which is definitive, or by visually checking the intestinal tract itself.

- Screening / monitoring programs involve visually examining the intestinal tract to check its healthy condition. It is undertaken as a routine matter every few years where there is cause for concern, when either (a) a genetic test has confirmed the risk or (b) a genetic test has not been undertaken for any reason so the actual risk is unknown. Screening and monitoring allows polyposis to be detected visually before it can become life-threatening.

- Treatment, typically surgery of some kind, is involved if polyposis has led to a large number of polyps, or a significant risk of cancer, or actual cancer.

Monitoring involves the provision of outpatient colonoscopy, and occasionally upper gastric tract esophagogastroduodenoscopy (EGD, to search for premalignant gastric or duodenal tumors), typically once every 1–3 years, and/or a genetic blood test to definitively confirm or deny susceptibility. A small number of polyps can often be excised (removed) during the procedure, if found, but if there are more severe signs or numbers, in patient surgery may be required.

NCBI states that when an individual is identified as having FAP, or the mutations resulting in FAP: "It is appropriate to evaluate the parents of an affected individual (a) with molecular genetic testing of APC if the disease-causing mutation is known in the proband [person first identified with the condition] or (b) for clinical manifestations of APC-associated polyposis conditions".

Genetic testing for mutations in DNA mismatch repair genes is expensive and time-consuming, so researchers have proposed techniques for identifying cancer patients who are most likely to be HNPCC carriers as ideal candidates for genetic testing. The Amsterdam Criteria (see below) are useful, but do not identify up to 30% of potential Lynch syndrome carriers. In colon cancer patients, pathologists can measure microsatellite instability in colon tumor specimens, which is a surrogate marker for DNA mismatch repair gene dysfunction. If there is microsatellite instability identified, there is a higher likelihood for a Lynch syndrome diagnosis. Recently, researchers combined microsatellite instability (MSI) profiling and immunohistochemistry testing for DNA mismatch repair gene expression and identified an extra 32% of Lynch syndrome carriers who would have been missed on MSI profiling alone. Currently, this combined immunohistochemistry and MSI profiling strategy is the most advanced way of identifying candidates for genetic testing for the Lynch syndrome.

Genetic counseling and genetic testing are recommended for families that meet the Amsterdam criteria, preferably before the onset of colon cancer.

Screening for colonic polyps as well as preventing them has become an important part of the management of the condition. Medical societies have established guidelines for colorectal screening in order to prevent adenomatous polyps and to minimize the chances of developing colon cancer. It is believed that some changes in the diet might be helpful in preventing polyps from occurring but there is no other way to prevent the polyps from developing into cancerous growths than by detecting and removing them.

According to the guidelines established by the American Cancer Society, individuals who reach the age of 50 should perform an occult blood test yearly. Colon polyps as they grow can sometimes cause bleeding within the intestine, which can be detected with the help of this test. Also, persons in their 50s are recommended to have flexible sigmoidoscopies performed once in 3 to 5 years to detect any abnormal growth which could be an adenomatous polyp. If adenomatous polyps are detected during this procedure, it is most likely that the patient will have to undergo a colonoscopy. Medical societies recommend colonoscopies every ten years starting at age 50 as a necessary screening practice for colon cancer. The screening provides an accurate image of the intestine and also allows the removal of the polyp, if found. Once an adenomatous polyp is identified during colonoscopy, there are several methods of removal including using a snare or a heating device. Colonoscopies are preferred over sigmoidoscopies because they allow the examination of the entire colon; a very important aspect, considering that more than half of the colonic polyps occur in the upper colon, which is not reached during sigmoidoscopies.

It has been statistically demonstrated that screening programs are effective in reducing the number of deaths caused by colon cancer due to adenomatous polyps. While there are risks of complications associated with colonoscopies, those risks are extremely low at approximately 0.35 percent. For comparison, the lifetime risk of developing colon cancer is around 6 percent. As there is a small likelihood of recurrence, surveillance after polyp removal is recommended.

In the United States screening is typically recommended between the age of 50 and 75 years. For those between 76 and 85 years of age the decision to screen should be individualized. A number of screening methods can be used including stool based tests every 3 years, sigmoidoscopy every 5 years and colonoscopy every 10 years. For those at high risk, screenings usually begin at around 40. It is unclear which of these two methods is better. Colonoscopy may find more cancers in the first part of the colon but is associated with greater cost and more complications. For people with average risk who have had a high-quality colonoscopy with normal results, the American Gastroenterological Association does not recommend any type of screening in the 10 years following the colonoscopy. For people over 75 or those with a life expectancy of less than 10 years, screening is not recommended. It takes about 10 years after screening for one out of a 1000 people to benefit.

In Canada, among those 50 to 75 at normal risk, fecal immunochemical testing or FOBT is recommended every two years or sigmoidoscopy every 10 years. Colonoscopy is less preferred.

Some countries have national colorectal screening programs which offer FOBT screening for all adults within a certain age group, typically starting between age 50 and 60. Examples of countries with organised screening include the United Kingdom, Australia and the Netherlands.

The following are the Amsterdam criteria in identifying high-risk candidates for molecular genetic testing:

"Amsterdam Criteria (all bullet points must be fulfilled):"

- Three or more family members with a confirmed diagnosis of colorectal cancer, one of whom is a first degree (parent, child, sibling) relative of the other two

- Two successive affected generations

- One or more colon cancers diagnosed under age 50 years

- Familial adenomatous polyposis (FAP) has been excluded

"Amsterdam Criteria II (all bullet points must be fulfilled):"

- Three or more family members with HNPCC-related cancers, one of whom is a first-degree relative of the other two

- Two successive affected generations

- One or more of the HNPCC-related cancers diagnosed under age 50 years

- Familial adenomatous polyposis (FAP) has been excluded

Colorectal polyps can be detected using a faecal occult blood test, flexible sigmoidoscopy, colonoscopy, virtual colonoscopy, digital rectal examination, barium enema or a pill camera.

Malignant potential is associated with

- degree of dysplasia

- Type of polyp (e.g. villous adenoma):

- Tubular Adenoma: 5% risk of cancer

- Tubulovillous adenoma: 20% risk of cancer

- Villous adenoma: 40% risk of cancer

- Size of polyp:

- <1 cm =<1% risk of cancer

- 1 cm=10% risk of cancer

- 2 cm=15% risk of cancer

Normally an adenoma which is greater than 0.5 cm is treated

Aspirin and celecoxib appear to decrease the risk of colorectal cancer in those at high risk. Aspirin is recommended in those who are 50 to 60 years old, do not have an increased risk of bleeding, and are at risk for cardiovascular disease to prevent colorectal cancer. It is not recommended in those at average risk. There is tentative evidence for calcium supplementation, but it is not sufficient to make a recommendation. Vitamin D intake and blood levels are associated with a lower risk of colon cancer.

Complete removal of a SSA is considered curative.

Several SSAs confer a higher risk of subsequently finding colorectal cancer and warrant more frequent surveillance. The surveillance guidelines are the same as for other colonic adenomas. The surveillance interval is dependent on (1) the number of adenomas, (2) the size of the adenomas, and (3) the presence of high-grade microscopic features.

Screening methods for colon cancer depend on detecting either precancerous changes such as certain kinds of polyps or on finding early and thus more treatable cancer. The extent to which screening procedures reduce the incidence of gastrointestinal cancer or mortality depends on the rate of precancerous and cancerous disease in that population. gFOBT (guaiac fecal occult blood test) and flexible sigmoidoscopy screening have each shown benefit in randomized clinical trials. Evidence for other colon cancer screening tools such as iFOBT (immunochemical fecal occult blood test) or colonoscopy is substantial and guidelines have been issued by several advisory groups but does not include randomized studies.

In 2009 the American College of Gastroenterology (ACG) suggest that colon cancer screening modalities that are also directly preventive by removing precursor lesions should be given precedence, and prefer a colonoscopy every 10 years in average-risk individuals, beginning at age 50. The ACG suggests that cancer detection tests such as any type of FOB are an alternative that is less preferred, and if a colonoscopy is declined, the FIT (fecal immunochemical test, or iFOBT) should be offered instead. Two other recent guidelines, from the US Multisociety Task Force (MSTF) and the US Preventive Services Task Force (USPSTF), while permitting immediate colonoscopy as an option, did not categorize it as preferred. The ACG and MSTF also included CT colonography every five years, and fecal DNA testing as considerations. All three recommendation panels recommended replacing any older low-sensitivity, guaiac-based fecal occult blood testing (gFOBT) with either newer high-sensitivity guaiac-based fecal occult blood testing (hs gFOBT) or fecal immunochemical testing (FIT). MSTF looked at six studies that compared high sensitivity gFOBT (Hemoccult SENSA) to FIT, and concluded that there was no clear difference in overall performance between these methods.

The American College of Gastroenterology has recommended the abandoning of gFOBT testing as a colorectal cancer screening tool, in favor of the fecal immunochemical test. Though the FIT test is preferred, even the guaiac FOB testing of average risk populations may have been sufficient to reduce the mortality associated with colon cancer by about 25%. With this lower efficacy, it was not always cost effective to screen a large population with gFOBT.

If colon cancer is suspected in an individual (such as in someone with an unexplained anemia) fecal occult blood tests may not be clinically helpful. If a doctor suspects colon cancer, more rigorous investigation is necessary, whether or not the test is positive.

In 2006, the Australian Government introduced the National Bowel Cancer Program which has been updated several times since; targeted screening will be done of all Australians aged over 50 to 74 by 2017–2018. Cancer Council Australia recommended that FOBT should be done every two years. Gradually government fund disbursement meant that some people are not yet eligible for the national program and should pay for a FOBT by themselves.

The Canadian Cancer Society recommends that men and women age 50 and over have a FOBT at least every 2 years.

In colon cancer screening, using only one sample of feces collected by a doctor performing a digital rectal examination is discouraged.

The use of the M2-PK Test is encouraged over gFOBT for routine screening as it may pick up tumors that are both bleeding and non bleeding. It is able to pick up 80 percent of colorectal cancer and 44 percent for adenoma > 1 centimeter, while gFOBT picks up 13 to 50 percent of colorectal cancers.

The serrated polyposis syndrome (SPS) is a relatively rare condition characterized by multiple and/or large serrated polyps of the colon. Diagnosis of this disease is made by the fulfillment of any of the World Health Organization’s (WHO) clinical criteria.

An extensive literature has examined the clinical value of FOBT in iron deficiency anemia.

Colon polyps are not commonly associated with symptoms. Occasionally rectal bleeding, and on rare occasions pain, diarrhea or constipation may occur because of colon polyps. Colon polyps are a concern because of the potential for colon cancer being present microscopically and the risk of benign colon polyps transforming over time into malignant ones. Since most polyps are asymptomatic, they are usually discovered at the time of colon cancer screening. Common screening methods are occult blood test, colonoscopy, sigmoidoscopy (usually flexible sigmoidoscopy, using a flexible endoscope, but more rarely the older rigid sigmoidoscopy, using a rigid endoscope), lower gastrointestinal series (barium enema), digital rectal examination (DRE), and virtual colonoscopy. The polyps are routinely removed at the time of colonoscopy either with a polypectomy snare (first description by P. Deyhle, Germany, 1970) or with biopsy forceps. If an adenomatous polyp is found with sigmoidoscopy or if a polyp is found with any other diagnostic modality, the patient must undergo colonoscopy for removal of the polyp(s). Even though colon cancer is usually not found in polyps smaller than 2.5 cm, all polyps found are removed since the removal of polyps reduces the future likelihood of developing colon cancer. When adenomatous polyps are removed, a repeat colonoscopy is usually performed in three to five years.

Most colon polyps can be categorized as sporadic.

Treatment:wide excision taking 8mm normal tissue as this is locally malignant. For recurrence radiotherapy is given

It is important to note that both barium enema and colonoscopy are contraindicated during acute episodes of diverticulitis, as the barium may leak out into the abdominal cavity, and colonoscopy can cause perforations of the bowel wall.

The U.S. Preventive Services Task Force (USPSTF) issues recommendations for various cancers:

- Strongly recommends cervical cancer screening in women who are sexually active and have a cervix at least until the age of 65.

- Recommend that Americans be screened for colorectal cancer via fecal occult blood testing, sigmoidoscopy, or colonoscopy starting at age 50 until age 75.

- Evidence is insufficient to recommend for or against screening for skin cancer, oral cancer, lung cancer, or prostate cancer in men under 75.

- Routine screening is not recommended for bladder cancer, testicular cancer, ovarian cancer, pancreatic cancer, or prostate cancer.

- Recommends mammography for breast cancer screening every two years from ages 50–74, but does not recommend either breast self-examination or clinical breast examination. A 2013 Cochrane review concluded that breast cancer screening by mammography had no effect in reducing mortality because of overdiagnosis and overtreatment.

Diagnosis is achieved mainly by plain and contrasted radiographical and ultrasound imaging. Colonic marker transit studies are useful to distinguish colonic inertia from functional outlet obstruction causes. In this test, the patient swallows a water-soluble bolus of radio-opaque contrast and films are obtained 1, 3 and 5 days later. Patients with colonic inertia show the marker spread throughout the large intestines, while patients with outlet obstruction exhibit slow accumulations of markers in some places. A colonoscopy can also be used to rule out mechanical obstructive causes. Anorectal manometry may help to differentiate acquired from congenital forms. Rectal biopsy is recommended to make a final diagnosis of Hirschsprung disease.

Unlike diagnostic efforts prompted by symptoms and medical signs, cancer screening involves efforts to detect cancer after it has formed, but before any noticeable symptoms appear. This may involve physical examination, blood or urine tests or medical imaging.

Cancer screening is not available for many types of cancers. Even when tests are available, they may not be recommended for everyone. "Universal screening" or "mass screening" involves screening everyone. "Selective screening" identifies people who are at higher risk, such as people with a family history. Several factors are considered to determine whether the benefits of screening outweigh the risks and the costs of screening. These factors include:

- Possible harms from the screening test: for example, X-ray images involve exposure to potentially harmful ionizing radiation

- The likelihood of the test correctly identifying cancer

- The likelihood that cancer is present: Screening is not normally useful for rare cancers.

- Possible harms from follow-up procedures

- Whether suitable treatment is available

- Whether early detection improves treatment outcomes

- Whether the cancer will ever need treatment

- Whether the test is acceptable to the people: If a screening test is too burdensome (for example, extremely painful), then people will refuse to participate.

- Cost

A high-fiber diet and fiber supplements are advisable to prevent constipation. The American Dietetic Association recommends 20–35 grams each day. Wheat bran has been shown to reduce intra colonic pressure.

The US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) says foods such as nuts, popcorn hulls, sunflower seeds, pumpkin seeds, caraway seeds, and sesame seeds have traditionally been labeled as problem foods for people with this condition; however, no scientific data exists to prove this hypothesis. The seeds in tomatoes, zucchini, cucumbers, strawberries, raspberries, and poppy seeds, are not considered harmful by the NIDDK.

One study found that nuts and popcorn do not contribute positively or negatively to patients with diverticulosis or diverticular complications.

Gardner syndrome, also known as Gardner's syndrome or familial colorectal polyposis, is an autosomal dominant form of polyposis characterized by the presence of multiple polyps in the colon together with tumors outside the colon. The extracolonic tumors may include osteomas of the skull, thyroid cancer, epidermoid cysts, fibromas, as well as the occurrence of desmoid tumors in approximately 15% of affected individuals.

Desmoid tumors are fibrous tumors which usually occur in the tissue covering the intestines and may be provoked by surgery to remove the colon. The countless polyps in the colon predispose to the development of colon cancer; if the colon is not removed, the chance of colon cancer is considered to be very significant. Polyps may also grow in the stomach, duodenum, spleen, kidneys, liver, mesentery and small bowel. In a small number of cases, polyps have also appeared in the cerebellum. Cancers related to Gardner syndrome commonly appear in the thyroid, liver and kidneys. The number of polyps increases with age, and hundreds to thousands of polyps can develop in the colon.

The syndrome was first described in 1951. There is no cure at this time, and in its more advanced forms, it is considered a terminal diagnosis with a life expectancy of 35–45 years; treatments are surgery and palliative care, although some chemotherapy has been tried with limited success.

Definitive diagnosis is made by suction biopsy of the distally narrowed segment. A histologic examination of the tissue would show a lack of ganglionic nerve cells. Diagnostic techniques involve anorectal manometry, barium enema, and rectal biopsy.

The suction rectal biopsy is considered the current international gold standard in the diagnosis of Hirschsprung's disease.

Radiologic findings may also assist with diagnosis. Cineanography (fluoroscopy of contrast medium passing anorectal region) assists in determining the level of the affected intestines.

The colorectal adenoma is a benign glandular tumor of the colon and the rectum. It is a precursor lesion of the colorectal adenocarcinoma (colon cancer).

Some morphological variants have been described:

- tubular adenoma

- tubulovillous adenoma

- villous adenoma

- sessile serrated adenoma (SSA)

The differential diagnosis includes colon cancer, inflammatory bowel disease, ischemic colitis, and irritable bowel syndrome, as well as a number of urological and gynecological processes.

People with the above symptoms are commonly studied with computed tomography, or CT scan. The CT scan is very accurate (98%) in diagnosing diverticulitis. In order to extract the most information possible about the patient's condition, thin section (5 mm) transverse images are obtained through the entire abdomen and pelvis after the patient has been administered oral and intravascular contrast. Images reveal localized colon wall thickening, with inflammation extending into the fat surrounding the colon. The diagnosis of acute diverticulitis is made confidently when the involved segment contains diverticula. CT may also identify patients with more complicated diverticulitis, such as those with an associated abscess. It may even allow for radiologically guided drainage of an associated abscess, sparing a patient from immediate surgical intervention.

Other studies, such as barium enema and colonoscopy, are contraindicated in the acute phase of diverticulitis because of the risk of perforation.

The severity of diverticulitis can be radiographically graded by the Hinchey Classification.