Breast cancer risk is elevated for defined fraction of lesions. Except for patients with a strong family history of breast cancer, where the risk is two-fold, nonproliferative lesions have no increased risk. Proliferative lesions also have approximately a 2-fold risk. In particular, atypical hyperplasia is associated with an increased risk of developing breast cancer. Atypical lobular hyperplasia is associated with the greatest risk, approximately 5-fold and especially high relative risk of developing premenopausal breast cancer. Atypical ductal hyperplasia is associated with 2.4-fold risk. In contrast, a New England Journal of Medicine article states that for women with a strong familial history of breast cancer, the risk of future breast cancer is roughly doubled, independent of histological status. The article further states "The relative risk of breast cancer for the cohort was 1.56 (95 percent confidence interval, 1.45 to 1.68), and this increased risk persisted for at least 25 years after biopsy. The relative risk associated with atypia was 4.24 (95 percent confidence interval, 3.26 to 5.41), as compared with a relative risk of 1.88 (95 percent confidence interval, 1.66 to 2.12) for proliferative changes without atypia and of 1.27 (95 percent confidence interval, 1.15 to 1.41) for nonproliferative lesions. The strength of the family history of breast cancer, available for 4808 women, was a risk factor that was independent of histologic findings. No increased risk was found among women with no family history and nonproliferative findings. In the first 10 years after the initial biopsy, an excess of cancers occurred in the same breast, especially in women with atypia."

It is not well understood whether the lesions are precursors of breast cancer or only indication of increased risk, for most types of lesions the chance of developing breast cancer is nearly the same in the affected and unaffected breast (side) indicating only coincidence of risk factors. For atypical lobular hyperplasia there is high incidence of ipsilateral breast cancers indicating a possible direct carcinogenetic link.

The estimated figures for the prevalence of fibrocystic breast changes in women over lifetime vary widely in the literature, with estimates ranging from about 30 to 60 % over about 50 to 60 % to about 60 to 75% of all women.

The condition is most common among women between 30 and 50 years of age.

Some women who have pain in one or both breasts may fear breast cancer. However, breast pain is not a common symptom of cancer. The great majority of breast cancer cases do not present with symptoms of pain, though breast pain in older women is more likely to be associated with cancer.

Breast atrophy is the normal or spontaneous atrophy or shrinkage of the breasts.

Breast atrophy commonly occurs in women during menopause when estrogen levels decrease. It can also be caused by hypoestrogenism and/or hyperandrogenism in women in general, such as in antiestrogen treatment for breast cancer, in polycystic ovary syndrome (PCOS), and in malnutrition such as that associated with eating disorders like anorexia nervosa or with chronic disease. It can also be an effect of weight loss.

In the treatment of gynecomastia in males and macromastia in women, and in hormone replacement therapy (HRT) for trans men, breast atrophy may be a desired effect.

Examples of treatment options for breast atrophy, depending on the situation/when appropriate, can include estrogens, antiandrogens, and proper nutrition or weight gain.

Approximately 10–25 percent of cases are estimated to result from the use of medications. This is known as non-physiologic gynecomastia. Medications known to cause gynecomastia include ketoconazole, cimetidine, gonadotropin-releasing hormone analogues, human growth hormone, human chorionic gonadotropin, 5α-Reductase inhibitors such as finasteride and dutasteride, estrogens such as those used in transgender women and men with prostate cancer, and antiandrogens such as bicalutamide, flutamide, and spironolactone. Medications that are probably associated with gynecomastia include calcium channel blockers such as verapamil, amlodipine, and nifedipine; risperidone, olanzapine, anabolic steroids, alcohol, opioids, efavirenz, alkylating agents, and omeprazole. Certain components of personal care products such as lavender or tea tree oil and certain supplements such as dong quai and "Tribulus terrestris" have been associated with gynecomastia.

Gynecomastia is thought to be caused by an altered ratio of estrogens to androgens mediated by an increase in estrogen production, a decrease in androgen production, or a combination of these two factors. Estrogen acts as a growth hormone to increase the size of male breast tissue. The cause of gynecomastia is unknown in around 25% of cases. Drugs are estimated to cause 10–25% of cases of gynecomastia.

Certain health problems in men such as liver disease, kidney failure or low testosterone can cause breast growth in men. Drugs and liver disease are the most common cause in adults. Other medications such as methadone, aldosterone antagonists (spironolactone and epelerenone), HIV medication, cancer chemotherapy, hormone treatment for prostate cancer, heartburn and ulcer medications, calcium channel blockers, antifungal medications such as ketoconazole, antibiotics such as metronidazole, tricyclic antidepressants such as amitriptyline, herbals such as lavender, tea tree oil, and dong quai are also known to cause gynecomastia. Phenothrin, an insecticide, possesses antiandrogen activity, and has been associated with gynecomastia.

It is estimated that 7% of women in the western world develop palpable breast cysts.

There is preliminary evidence that women with breast cysts may be at an increased risk of breast cancer, especially at younger ages.

In males, the occurrence of breast cysts is rare and may (but need not) be an indication of malignancy.

In the Western world, the typical age of menopause (last period from natural causes) is between 40 and 61 and the average age for last period is 51 years. The average age of natural menopause in Australia is 51.7 years. In India and the Philippines, the median age of natural menopause is considerably earlier, at 44 years.

In rare cases, a woman's ovaries stop working at a very early age, ranging anywhere from the age of puberty to age 40. This is known as premature ovarian failure and affects 1 to 2% of women by age 40.

Undiagnosed and untreated coeliac disease is a risk factor for early menopause. Coeliac disease can present with several non-gastrointestinal symptoms, in the absence of gastrointestinal symptoms, and most cases escape timely recognition and go undiagnosed, leading to a risk of long-term complications. A strict gluten-free diet reduces the risk. Women with early diagnosis and treatment of coeliac disease present a normal duration of fertile life span.

Women who have undergone hysterectomy with ovary conservation go through menopause on average 3.7 years earlier than the expected age. Other factors that can promote an earlier onset of menopause (usually 1 to 3 years early) are smoking cigarettes or being extremely thin.

About one percent of breast cancer develops in males. It is estimated that about 2,140 new cases are diagnosed annually in the United States (US) and about 300 in the United Kingdom (UK). The number of annual deaths in the US is about 440 (for 2016 "but fairly stable over the last 30 years"). In a study from India, eight out of 1,200 (0.7%) male cancer diagnoses in a pathology review represented breast cancer. Incidence of male breast cancer has been increasing which raises the probability of other family members developing the disease. The relative risk of breast cancer for a female with an affected brother is approximately 30% higher than for a female with an affected sister. The tumor can occur over a wide age range, but typically appears in males in their sixties and seventies.

Known risk factors include radiation exposure, exposure to female hormones (estrogen), and genetic factors. High estrogen exposure may occur by medications, obesity, or liver disease, and genetic links include a high prevalence of female breast cancer in close relatives. Chronic alcoholism has been linked to male breast cancer. The highest risk for male breast cancer is carried by males with Klinefelter syndrome. Male BRCA mutation carriers are thought to be at higher risk for breast cancer as well, with roughly 10% of male breast cancer cases carrying BRCA2 mutations, and BRCA1 mutation being in the minority.

There are case reports of gigantomastia occurring in infants as well.

The underlying cause of the rapidly growing breast connective tissue, resulting in gigantic proportions, has not been well-elucidated. However, proposed factors have included increased levels/expression of or heightened sensitivity to certain hormones (e.g., estrogen, progesterone, and prolactin) and/or growth factors (e.g., hepatic growth factor, insulin-like growth factor 1, and epidermal growth factor) in the breasts. Macromastic breasts are reported to be composed mainly of adipose and fibrous tissue, while glandular tissue remains essentially stable.

Macromastia occurs in approximately half of women with aromatase excess syndrome (a condition of hyperestrogenism). Hyperprolactinemia has been reported as a cause of some cases of macromastia. Macromastia has also been associated with hypercalcemia (which is thought to be due to excessive production of parathyroid hormone-related protein) and, rarely, systemic lupus erythematosus and pseudoangiomatous stromal hyperplasia. It is also notable that approximately two-thirds of women with macromastia are obese. Aside from aromatase (as in aromatase excess syndrome), at least two other genetic mutations (one in PTEN) have been implicated in causing macromastia.

A handful of drugs have been associated with gigantomastia, including penicillamine, bucillamine, neothetazone, ciclosporin, and indinavir.

Adipomastia, or lipomastia, also known colloquially as fatty breasts, is a condition defined as an excess of skin and adipose tissue in the breasts without true breast glandular tissue. It is commonly present in men with obesity, and is particularly apparent in men who have undergone massive weight loss. A related/synonymous term is pseudogynecomastia. The condition is different and should be distinguished from gynecomastia ("women's breasts"), which involves true glandular breast development in a male. The two conditions can usually be distinguished easily by palpation to check for the presence of glandular tissue. Another difference between the conditions is that breast pain/tenderness does not occur in pseudogynecomastia. Sometimes, gynecomastia and pseudogynecomastia are present together; this is related to the fact that fat tissue expresses aromatase, the enzyme responsible for the synthesis of estrogen, and estrogen is produced to a disproportionate extent in men with excessive amounts of fat, resulting in simultaneous glandular enlargement.

Cyclical breast pain (cyclical mastalgia) is often associated with fibrocystic breast changes or duct ectasia and thought to be caused by changes of prolactin response to thyrotropin. Some degree of cyclical breast tenderness is normal in the menstrual cycle, and is usually associated with menstruation and/or premenstrual syndrome (PMS).

Noncyclical breast pain has various causes and is harder to diagnose. Noncyclical pain has frequently its root cause outside the breast. Some degree of non-cyclical breast tenderness can normally be present due to hormonal changes in puberty (both in girls and boys), in menopause and during pregnancy. After pregnancy, breast pain can be caused by breastfeeding. Other causes of non-cyclical breast pain include alcoholism with liver damage (likely due to abnormal steroid metabolism), mastitis and medications such as digitalis, methyldopa (an antihypertensive), spironolactone, certain diuretics, oxymetholone (an anabolic steroid), and chlorpromazine (a typical antipsychotic). Also, shingles can cause a painful blistering rash on the skin of the breasts.

Menopause confers:

- A possible but contentious increased risk of atherosclerosis. The risk of acute myocardial infarction and other cardiovascular diseases rises sharply after menopause, but the risk can be reduced by managing risk factors, such as tobacco smoking, hypertension, increased blood lipids and body weight.

- Increased risk of osteopenia, osteoporosis, and accelerated lung function decline.

Women who experience menopause before 45 years of age have an increased risk of heart disease, death, and impaired lung function.

The development of breast cysts may be prevented to some degree, according to the majority of the specialists. The recommended measures one is able to take in order to avoid the formation of the cysts include practicing good health and avoiding certain medications, eating a balanced diet, taking necessary vitamins and supplements, getting exercise, and avoiding stress.

Although caffeine consumption does not have a scientifically proved connection with the process of cyst development, many women claim that their symptoms are relieved if avoiding it. Some doctors recommend reducing the amount of caffeine in one's diet in terms of both beverages and foods (such as chocolate). Also reducing salt intake may help in alleviating the symptoms of breast cysts, although, again, there is no scientific linkage between these two. Excessive sugar consumption as well as undetected food allergies, such as to gluten or lactose, may also contribute to cyst development.

It occurs in all adult age groups. While the majority of patients are between 40 and 59 years old, age predilection is much less pronounced than in noninflammatory breast cancer. The overall rate is 1.3 cases per 100000, black women (1.6) have the highest rate, Asian and Pacific Islander women the lowest (0.7) rates.

Most known breast cancer risk predictors do not apply for inflammatory breast cancer. It may be slightly associated with cumulative breast-feeding duration.

Smoking tobacco appears to increase the risk of breast cancer, with the greater the amount smoked and the earlier in life that smoking began, the higher the risk. In those who are long-term smokers, the risk is increased 35% to 50%. A lack of physical activity has been linked to about 10% of cases. Sitting regularly for prolonged periods is associated with higher mortality from breast cancer. The risk is not negated by regular exercise, though it is lowered.

There is an association between use of hormonal birth control and the development of premenopausal breast cancer, but whether oral contraceptives use may actually cause premenopausal breast cancer is a matter of debate. If there is indeed a link, the absolute effect is small. Additionally, it is not clear if the association exists with newer hormonal birth controls. In those with mutations in the breast cancer susceptibility genes "BRCA1" or "BRCA2", or who have a family history of breast cancer, use of modern oral contraceptives does not appear to affect the risk of breast cancer.

The association between breast feeding and breast cancer has not been clearly determined; some studies have found support for an association while others have not. In the 1980s, the abortion–breast cancer hypothesis posited that induced abortion increased the risk of developing breast cancer. This hypothesis was the subject of extensive scientific inquiry, which concluded that neither miscarriages nor abortions are associated with a heightened risk for breast cancer.

A number of dietary factors have been linked to the risk for breast cancer. Dietary factors which may increase risk include a high fat diet, high alcohol intake, and obesity-related high cholesterol levels. Dietary iodine deficiency may also play a role. Evidence for fiber is unclear. A 2015 review found that studies trying to link fiber intake with breast cancer produced mixed results. In 2016 a tentative association between low fiber intake during adolescence and breast cancer was observed.

Other risk factors include radiation and shift-work. A number of chemicals have also been linked, including polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and organic solvents Although the radiation from mammography is a low dose, it is estimated that yearly screening from 40 to 80 years of age will cause approximately 225 cases of fatal breast cancer per million women screened.

In some population studies moderate alcohol consumption is associated with increase the breast cancer risk.

In contrast, research by the Danish National Institute for Public Health, comprising 13,074 women aged 20 to 91 years, found that moderate drinking had virtually no effect on breast cancer risk.

Studies that control for screening incidence show no association with moderate drinking and breast cancer, e.g.. Moderate drinkers tend to screen more which results in more diagnoses of breast cancer, including mis-diagnoses. A recent study of 23 years of breast cancer screening in the Netherlands concluded that 50% of diagnoses were over-diagnoses.

Studies suggest that drinking alcohol during pregnancy may affect the likelihood of breast cancer in daughters. "For women who are pregnant, ingestion of alcohol, even in moderation, may lead to elevated circulating oestradiol levels, either through a reduction of melatonin or some other mechanism. This may then affect the developing mammary tissue such that the lifetime risk of breast cancer is raised in their daughters."

Risk factors can be divided into two categories:

- "modifiable" risk factors (things that people can change themselves, such as consumption of alcoholic beverages), and

- "fixed" risk factors (things that cannot be changed, such as age and biological sex).

The primary risk factors for breast cancer are being female and older age. Other potential risk factors include genetics, lack of childbearing or lack of breastfeeding, higher levels of certain hormones, certain dietary patterns, and obesity. Recent studies have indicated that exposure to light pollution is a risk factor for the development of breast cancer.

Among women worldwide, breast cancer is the most common cause of cancer death. Breast self-examination (BSE) is an easy but unreliable method for finding possible breast cancer. Factors that appear to be implicated in decreasing the risk of, early diagnosis of. or recurrence of breast cancer are regular breast examinations by health care professionals, regular mammograms, self-examination of breasts, healthy diet, and exercise to decrease excess body fat.

Adjusted for age and stage the prognosis for breast cancer in males is similar to that in females. Prognostically favorable are smaller tumor size and absence or paucity of local lymph node involvement. Hormonal treatment may be associated with hot flashes and impotence.

Any deformity of the breasts is only apparent during puberty and this may lead to psychosexual problems with girls in very early puberty being affected psychologically due to the unusual shape of the breast. Surgical papers about the techniques useful in correcting tubular breasts note that

even when results are not perfect, the psychological impact of treatment is immense, with notable improvements in self-esteem to the level where the person engages in normal social activities.

Presentations of low estrogen levels include hot flashes, headaches, lowered libido, and breast atrophy. Reduced bone density leading to secondary osteoporosis and atrophic changes such as pH change in the vagina is also linked to hypoestrogenism.

Low levels of estrogen can lead to dyspareunia and limited genital arousal because of changes in the four layers of the vaginal wall.

Hypoestrogenism is also considered one of the major risk factors for developing uncomplicated urinary tract infections (UTIs) in postmenopausal women who do not take hormone replacement therapy.

Tuberous breasts (or tubular breasts) are a result of a congenital abnormality of the breasts which can occur in both men and women (also see Hypoplasia), one breast or both. During puberty breast development is stymied and the breasts fail to develop normally and fully. The exact cause of this is as yet unclear, however, a study in 2011 of the cells in the breasts of both males and females with tubular breasts suggested a genetic link in a disorder of collagen deposition. The condition is thought to affect one to five per cent of breast augmentation patients; however, the proportion of the general population affected is unknown as surgery is not always sought.