Carcinoid tumors are noncancerous (benign) or cancerous (malignant) growths that produce excessive amounts of hormonelike substances. Carcinoid tumors usually originate in hormone-producing cells that line the small intestine or other cells of the digestive tract. They can also occur in the pancreas, testes, ovaries, or lungs. Carcinoid tumors can produce an excess of hormonelike substances, such as serotonin, bradykinin, histamine, and prostaglandins. Excess levels of these substances can sometimes result in a diverse set of symptoms called carcinoid syndrome.