Parts of the body where gastrointestinal carcinoid tumors form; drawing of the gastrointestinal tract showing the stomach, small intestine (including the duodenum, jejunum, and ileum), appendix, colon, and rectum.
Gastrointestinal carcinoid tumors form in the lining of the gastrointestinal tract, most often in the appendix, small intestine, or rectum.