Radiation therapy directed at the abdomen may damage the digestive tract, the type and extent of injury depending on the dose of the radiation and the radiation sensitivity of the gut. Characteristic early changes are manifest in the mucosa of the gut: for later ulceration, changes in the collagen tissues and particularly in the vascular channels occur. This paper describes and characterizes injuries to the esophagus, stomach, small intestine and colon. It emphasizes the importance of recognizing radiation-induced damage to the gut which may occur early or late after radiation.