While most carcinoids are asymptomatic through the natural lifetime and are discovered only upon surgery for unrelated reasons (so-called "coincidental carcinoids"), all carcinoids are considered to have malignant potential.

About 10% of carcinoids secrete excessive levels of a range of hormones, most notably serotonin (5-HT), causing:

- Flushing (serotonin itself does not cause flushing). Potential causes of flushing in carcinoid syndrome include bradykinins, prostaglandins, tachykinins, substance P, and/or histamine, diarrhea, and heart problems. Because of serotonin's growth-promoting effect on cardiac myocytes,[14] a serotonin-secreting carcinoid tumour may cause a tricuspid valve disease syndrome, due to the proliferation of myocytes onto the valve.

- Diarrhea

- Wheezing

- Abdominal cramping

- Peripheral edema

The outflow of serotonin can cause a depletion of tryptophan leading to niacin deficiency. Niacin deficiency, also known as pellagra, is associated with dermatitis, dementia, and diarrhea.

This constellation of symptoms is called "carcinoid syndrome" or (if acute) "carcinoid crisis". Occasionally, haemorrhage or the effects of tumor bulk are the presenting symptoms. The most common originating sites of carcinoid is the small bowel, particularly the ileum; carcinoid tumors are the most common malignancy of the appendix. Carcinoid tumors may rarely arise from the ovary or thymus.

They are most commonly found in the midgut at the level of the ileum or in the appendix. The next most common affected area is the respiratory tract, with 28% of all cases — per PAN-SEER data (1973 – 1999). The rectum is also a common site.

Some PanNETs do not cause any symptoms, in which case they may be discovered incidentally on a CT scan performed for a different purpose. Symptoms such as abdominal or back pain or pressure, diarrhea, indigestion, or yellowing of the skin and whites of the eyes can arise from the effects of a larger PanNET tumor, either locally or at a metastasis. About 40% of PanNETS have symptoms related to excessive secretion of hormones or active polypeptides and are accordingly labeled as "functional"; the symptoms reflect the type of hormone secreted, as discussed below. Up to 60% of PanNETs are nonsecretory or nonfunctional, in which there is no secretion, or the quantity or type of products, such as pancreatic polypeptide (PPoma), chromogranin A, and neurotensin, do not cause a clinical syndrome although blood levels may be elevated. In total, 85% of PanNETs have an elevated blood marker.

Functional tumors are often classified by the hormone most strongly secreted, for example:

- gastrinoma: the excessive gastrin causes Zollinger–Ellison syndrome (ZES) with peptic ulcers and diarrhea

- insulinoma: hypoglycemia occurs with concurrent elevations of insulin, proinsulin and C peptide

- glucagonoma: the symptoms are not all due to glucagon elevations, and include a rash, sore mouth, altered bowel habits, venous thrombosis, and high blood glucose levels

- VIPoma, producing excessive vasoactive intestinal peptide, which may cause profound chronic watery diarrhea and resultant dehydration, hypokalemia, and achlorhydria (WDHA or pancreatic cholera syndrome)

- somatostatinoma: these rare tumors are associated with elevated blood glucose levels, achlorhydria, cholelithiasis, and diarrhea

- less common types include ACTHoma, CRHoma, calcitoninoma, GHRHoma, GRFoma, and parathyroid hormone–related peptide tumor

In these various types of functional tumors, the frequency of malignancy and the survival prognosis have been estimated dissimilarly, but a pertinent accessible summary is available.

GISTs may present with trouble swallowing, gastrointestinal bleeding, or metastases (mainly in the liver). Intestinal obstruction is rare, due to the tumor's outward pattern of growth. Often, there is a history of vague abdominal pain or discomfort, and the tumor has become rather large by time the diagnosis is made.

Carcinoid (also carcinoid tumor) is a slow-growing type of neuroendocrine tumor originating in the cells of the neuroendocrine system. In some cases, metastasis may occur. Carcinoid tumors of the midgut (jejunum, ileum, appendix, and cecum) are associated with carcinoid syndrome.

Carcinoid tumors are the most common malignant tumor of the appendix, but they are most commonly associated with the small intestine, and they can also be found in the rectum and stomach. They are known to grow in the liver, but this finding is usually a manifestation of metastatic disease from a primary carcinoid occurring elsewhere in the body. They have a very slow growth rate compared to most malignant tumors. The median age at diagnosis for all patients with neuroendocrine tumors is 63 years.

PanNETs are sometimes abbreviated as PETs or PNETs: such use should not to be confused with the primitive neuroectodermal tumor (PNET).

The majority of PanNETs are benign, while some are malignant. The World Health Organization (WHO) classification scheme places neuroendocrine tumors into three main categories, which emphasize the tumor grade rather than the anatomical origin. In practice, those tumors termed well or intermediately differentiated PanNETs in the WHO scheme are sometimes called "islet cell tumors." The high grade subtype, termed neuroendocrine cancer (NEC) in the WHO scheme, is synonymous with "islet cell carcinoma".

The World Health Organization (WHO) classification scheme places neuroendocrine tumors into three main categories, which emphasize the tumor grade rather than the anatomical origin:

- well-differentiated neuroendocrine tumours, further subdivided into tumors with benign and those with uncertain behavior

- well-differentiated (low grade) neuroendocrine carcinomas with low-grade malignant behavior

- poorly differentiated (high grade) neuroendocrine carcinomas, which are the large cell neuroendocrine and small cell carcinomas.

Additionally, the WHO scheme recognizes mixed tumors with both neuroendocrine and epithelial carcinoma features, such as goblet cell cancer, a rare gastrointestinal tract tumor.

Placing a given tumor into one of categories depends on well-defined histological features: size, lymphovascular invasion, mitotic counts, Ki-67 labelling index, invasion of adjacent organs, presence of metastases and whether they produce hormones.

Conceptually, there are two main types of NET within this category: those which arise from the gastrointestinal (GI) system and those that arise from the pancreas. In usage, the term "carcinoid" has often been applied to both, although sometimes it is restrictively applied to NETs of GI origin (as herein), or alternatively to those tumors which secrete functional hormones or polypeptides associated with clinical symptoms, as discussed.

GISTs are tumors of connective tissue, i.e. sarcomas; unlike most gastrointestinal tumors, they are nonepithelial. About 70% occur in the stomach, 20% in the small intestine and less than 10% in the esophagus. Small tumors are generally benign, especially when cell division rate is slow, but large tumors disseminate to the liver, omentum and peritoneal cavity. They rarely occur in other abdominal organs.

GISTs are thought to arise from interstitial cells of Cajal (ICC), that are normally part of the autonomic nervous system of the intestine. They serve a pacemaker function in controlling motility.

Appendix cancer or appendiceal cancers are rare malignancies of the vermiform appendix.

Gastrointestinal stromal tumors are rare tumors with malignant potential. Primary lymphomas can occur in the appendix. Breast cancer, colon cancer, and tumors of the female genital tract may metastasize to the appendix.

Most paragangliomas are either asymptomatic or present as a painless mass. While all contain neurosecretory granules, only in 1–3% of cases is secretion of hormones such as catecholamines abundant enough to be clinically significant; in that case manifestations often resemble those of pheochromocytomas (intra-medullary paraganglioma).

GCCs have an aggressive course compared to other appendiceal neuroendocrine tumours.

About 85% of paragangliomas develop in the abdomen; only 12% develop in the chest and 3% in the head and neck region (the latter are the most likely to be symptomatic). While most are single, rare multiple cases occur (usually in a hereditary syndrome). Paragangliomas are described by their site of origin and are often given special names:

- Carotid paraganglioma (carotid body tumor): Is the most common of the head and neck paragangliomas. It usually presents as a painless neck mass, but larger tumors may cause cranial nerve palsies, usually of the vagus nerve and hypoglossal nerve.

- Organ of Zuckerkandl: A collection of paraganglia near the bifurcation of the aorta, comprising a small mass of neural crest-derived chromaffin cells. Serves as a common origin of abdominal paragangliomas.

- Glomus tympanicum and Glomus jugulare: Both commonly present as a middle ear mass resulting in tinnitus (in 80%) and hearing loss (in 60%). The cranial nerves of the jugular foramen may be compressed, resulting swallowing difficulty, or ipsilateral weakness of the upper trapezius and sternocleiodomastoid muscles (from compression of the spinal accessory nerve). These patients present with a reddish bulge behind an intact ear drum. This condition is also known as the "Red drum". On application of pressure to the external ear canal with the help of a pneumatic ear speculum the mass could be seen to blanch. This sign is known as "Brown's sign". A deficient bony plate along the tympanic portion of the internal carotid artery (aberrant ICA) is a normal variant and can be mistaken with glomus jugulare.

- Vagal paraganglioma: These are the least common of the head and neck paragangliomas. They usually present as a painless neck mass, but may result in dysphagia and hoarseness.

- Pulmonary paraganglioma: These occur in the lung and may be either single or multiple.

- Other sites: Rare sites of involvement are the larynx, nasal cavity, paranasal sinuses, thyroid gland, and the thoracic inlet, as well as the bladder in extremely rare cases.

GP consist of three components (1) ganglion cells, (2) epithelioid cells (neuroendocrine-like), and (3) spindle cells (schwannoma-like). The microscopic differential diagnosis includes poorly differentiated carcinoma, neuroendocrine tumour and paraganglioma.

GPs may be sporadic or arise in the context neurofibromatosis type 1.

The most common presentation is gastrointestinal bleed (~45% of cases), followed by abdominal pain (~43% of cases) and anemia (~15% of cases).

The goblet cell carcinoid, abbreviated GCC and also known as crypt cell carcinoma and neuroendocrine tumour with goblet cell differentiation, is a rare biphasic gastrointestinal tract tumour that consists of a neuroendocrine component and a conventional carcinoma, histologically arising from Paneth cells.

The symptoms of choroid plexus carcinoma are similar to those of other brain tumors. They include:

- Persistent or new onset headaches

- Macrocephaly or bulging fontanels in infants.

- Loss of appetite (refusal to take food in infants)

- Papilledema

- Nausea and emesis

- Ataxia

- Strabismus

- Developmental delays

- Altered mental status

Esthesioneuroblastoma will first frequently present as a nasal mass. The most common signs and symptoms of esthesioneuroblastoma are nasal obstruction (70%) and epistaxis (50%). Less common symptoms include hyposmia (loss of smell), headache, rhinorrhea, vision loss, proptosis, facial pain, diplopia (double vision), masses in the neck and changes in mental status. Esthesioneuroblastoma occurs in the upper nasal cavity, near the optic nerves and optic chiasm. Thus, tumor growth can impinge nerve function and result in vision loss and diplopia. As the tumor metastasizes to the oral cavity, there can be tooth pain and tooth mobility.

Adenocarcinoma (; plural adenocarcinomas or adenocarcinomata ) is a type of cancerous tumor that can occur in several parts of the body. It is defined as neoplasia of epithelial tissue that has glandular origin, glandular characteristics, or both. Adenocarcinomas are part of the larger grouping of carcinomas, but are also sometimes called by more precise terms omitting the word, where these exist. Thus invasive ductal carcinoma, the most common form of breast cancer, is adenocarcinoma but does not use the term in its name—however, esophageal adenocarcinoma does to distinguish it from the other common type of esophageal cancer, esophageal squamous cell carcinoma. Several of the most common forms of cancer are adenocarcinomas, and the various sorts of adenocarcinoma vary greatly in all their aspects, so that few useful generalizations can be made about them.

In the most specific usage (narrowest sense), the glandular origin or traits are exocrine; endocrine gland tumors, such as a VIPoma, an insulinoma, or a pheochromocytoma, are typically not referred to as adenocarcinomas but rather are often called neuroendocrine tumors. Epithelial tissue sometimes includes, but is not limited to, the surface layer of skin, glands, and a variety of other tissue that lines the cavities and organs of the body. Epithelial tissue can be derived embryologically from any of the germ layers (ectoderm, endoderm, or mesoderm). To be classified as adenocarcinoma, the cells do not necessarily need to be part of a gland, as long as they have secretory properties. Adenocarcinoma is the malignant counterpart to adenoma, which is the benign form of such tumors. Sometimes adenomas transform into adenocarcinomas, but most do not.

Well differentiated adenocarcinomas tend to resemble the glandular tissue that they are derived from, while poorly differentiated adenocarcinomas may not. By staining the cells from a biopsy, a pathologist can determine whether the tumor is an adenocarcinoma or some other type of cancer. Adenocarcinomas can arise in many tissues of the body owing to the ubiquitous nature of glands within the body, and, more fundamentally, to the potency of epithelial cells. While each gland may not be secreting the same substance, as long as there is an exocrine function to the cell, it is considered glandular and its malignant form is therefore named adenocarcinoma.

Pulmonary neuroendocrine tumor are classified according to tumoral grade:

- Low grade pulmonary neuroendocrine tumor: Typical pulmonary carcinoid tumour (TC; low-grade);

- Intermediate-grade pulmonary neuroendocrine tumor: Atypical pulmonary carcinoid tumour (AC; intermediate-grade)

- High-grade pulmonary neuroendocrine tumor

- Small cell lung cancer (SCLC)

- Large cell neuroendocrine carcinoma (LCNEC of the lung)

Low-grade nodular neuroendocrine proliferations ≥ 0.5 cm are classified as carcinoid tumors and smaller ones are called pulmonary tumorlets.

When neuroendocrine cell hyperplasia and tumorlets are extensive, they represent the rare preinvasive lesion for carcinoids known as "diffuse idiopathic pulmonary neuroendocrine cell hyperplasia".

Both LCNEC and SCLC can demonstrate histologic heterogeneity with other major histologic types of lung carcinoma, such as pulmonary adenocarcinoma or pulmonary squamous cell carcinoma, but is not characteristic of TC or AC.

Pulmonary neuroendocrine tumors are neuroendocrine tumors localized to the lung: bronchus or pulmonary parenchyma.

Pulmonary neuroendocrine tumors include a spectrum of tumors from the low-grade typical pulmonary carcinoid tumor and intermediate-grade atypical pulmonary carcinoid tumor to the high-grade pulmonary large cell neuroendocrine carcinoma (LCNEC) and pulmonary small cell carcinoma (SCLC), with significant clinical, epidemiologic and genetic differences.

This tumor only affects the outer 1/3 to 1/2 of the external auditory canal as a primary site. If this area is not involved, the diagnosis should be questioned. The most common tumor type is ceruminous adenoid cystic carcinoma and ceruminous adenocarcinoma, NOS.

An adrenal tumor or adrenal mass is any benign or malignant neoplasms of the adrenal gland, several of which are notable for their tendency to overproduce endocrine hormones. Adrenal cancer is the presence of malignant adrenal tumors, and includes neuroblastoma, adrenocortical carcinoma and some adrenal pheochromocytomas. Most adrenal pheochromocytomas and all adrenocortical adenomas are benign tumors, which do not metastasize or invade nearby tissues, but may cause significant health problems by unbalancing hormones.

Pain is the most common symptom, followed by either sensorineural or conductive hearing loss, tinnitus or drainage (discharge). A mass lesion may be present, but it is often slow growing.

Pineoblastoma (also pinealoblastoma) is a malignant tumor of the pineal gland. A pineoblastoma is a supratentorial midline primitive neuroectodermal tumor.

Pineoblastoma may occur in patients with hereditary uni- or bilateral retinoblastoma. When retinoblastoma patients present with pineoblastoma this is characterized as "trilateral retinoblastoma". Up to 5% of patients with hereditary retinoblastoma are at risk of developing trilateral retinoblastoma. Prognosis of patients with trilateral retinoblastoma is dismal, only a few patients have survived more than 5 years after diagnosis; all survivors were diagnosed with small tumors in a subclinical stage. Recent advances in (high-dose) chemotherapy treatment regimens and early detection have improved survival of patients with trilateral retinoblastoma to up to 50%.

Hereditary nonpolyposis colorectal cancer (HNPCC) or Lynch syndrome is an autosomal dominant genetic condition that has a high risk of colon cancer as well as other cancers including endometrial cancer (second most common), ovary, stomach, small intestine, hepatobiliary tract, upper urinary tract, brain, and skin. The increased risk for these cancers is due to inherited mutations that impair DNA mismatch repair. It is a type of cancer syndrome.