D. Walsh described the first patient with radiation-induced enteropathy in 1897, two years after Roentgen's description of ionizing radiation. Radiation therapy is highly useful for the control of some cancers, and with the use of higher doses, acute enteritis and its chronic sequelae appear. The sites of more severe injury after pelvic irradiation are in the lower sigmoid colon and upper rectum, probably because of their proximity to the field of irradiation. Acute radiation changes are a function of the rate and duration of time over which radiation is applied, whereas chronic damage appears to be better correlated with the total radiation dose and the volume of the bowel irradiated. This chapter reviews the use of elemental diets in the prophylaxis and therapy for intestinal lesions. Morphological recovery following acute radiation in mice is superior when animals are fed hydrolyzed casein, compared with those fed whole casein or Purina mouse chow.