Little is known about the causes of gastrointestinal carcinoid tumors, although they are known to start out very small and spread very slowly.

Certain factors appear to increase the risk for developing gastrointestinal carcinoid tumors, including:

• Family history of multiple endocrine neoplasia type 1 syndrome (MEN1 syndrome). This hereditary condition creates a high risk of developing tumors in the pituitary gland, parathyroid gland and pancreas. MEN1 syndrome is mostly like to cause carcinoid tumors in the stomach.
  
  
• Conditions that affect the stomach’s production of stomach acid. These include atrophic gastritis, pernicious anemia or Zollinger-Ellison syndrome.
  
  
• Smoking. A recent European study suggested that smoking may double the risk of developing carcinoid tumor of the small intestine.
  
  
• Race. Carcinoid tumors are slightly more common among African Americans than Caucasians.
  
  
• Age. Most carcinoid tumors are found in people above age 50, although carcinoid tumors in the appendix may be more common in younger adults.