CARCINOID SYNDROME - A constellation of symptoms associated with a serotonin-secreting neoplasm known as carcinoid tumor. These tumors may grow anywhere in the gastrointestinal tract (and in the lungs) with approximately 90% in the appendix. The remainder occur in the ileum, stomach, colon, or rectum. Symptoms include flushing, rapid pulse, facial swelling, swelling around the eyes, low blood pressure, abdominal pain, weight loss, and diarrhea.