A paraneoplastic syndrome comprising the signs and symptoms that occur secondary to carcinoid tumors.
Carcinoid tumors most frequently occur in the ileum and appendix with a small proportion occurring in the colon and rectum; about 10% of primary carcinoid tumors are pulmonary.

Carcinoid syndrome comprises cutaneous flushing, diarrhea, bronchoconstriction, and right heart failure.
The symptoms are caused by serotonin as well as a host of other substances released from the tumors, including kallikrein, serotonin, substance P and other neuropeptides, prostaglandins, histamine, and catecholamines. Pulmonary carcinoid tumors, but not those from the gastrointestinal tract, may also secrete ACTH and ADH. The cardiac disease involves a characteristic deposition on the valves of the right side of the heart, causing tricuspid incompetence and pulmonary stenosis. The carcinoid syndrome has usually been present for more than five years before carcinoid heart disease becomes clinically evident.

Woodruff R. Palliative medicine evidence-based symptomatic and supportive care for patients with advanced cancer. 4th ed. Oxford University Press, 2004. p. 419.