There is no histologic difference between carcinoids arising in various sites or between metastasizing and nonmetastasizing lesions. Carcinoid tumors are neuroendocrine tumors composed of uniform, round, or polygonal cells. Immunohistochemistry reveals the presence of neuron-specific enolase (NSE) and chromogranin. Electron microscopy shows neurosecretory granules. Morphologic features suggesting malignancy (cellular pleomorphism, hyperchromatic nuclei, necrosis, high mitotic activity) can occasionally be seen. Such cases are designated as atypical carcinoids and usually have an aggressive clinical course. Carcinoids are classified by their embryologic relationship to the foregut (the anterior part of the alimentary canal, from the mouth to the intestine or to the entrance of the bile duct), midgut (the middle part of the alimentary canal, from the stomach or entrance of the bile duct to or including the large intestine), or hindgut (the posterior part of the alimentary canal, including the rectum and sometimes the large intestine), which is correlated with clinical behavior and the secretion or nonsecretion of various neuroendocrine peptides. Proximal carcinoids may secrete histamine-like peptides causing a pink flush and bronchoconstriction. Peptide secretions of midgut carcinoids cause a cyanotic (purplish) flush, diarrhea, and hypotension. Hindgut carcinoids usually do not secrete syndrome-producing peptides.