It is hoped that this study of intestinal damage produced by 1,000-kv. roentgen irradiation will be of value for future radiotherapy. The increasing availability of supervoltage irradiation will help the radiation therapist destroy many abdominal tumors which he has been content to palliate. In so doing he may have to exceed the tolerance of the normal intestine, especially in treating carcinoma of the ovary or kidney, advanced carcinoma of the cervix, and retroperitoneal metastases from testicular tumors. A typical recent study of radiation injury to the intestine is that of Hock, Rodri-gues, Hamann, and Palmer (1), based on 344 cases of carcinoma of the female reproductive system. Analysis of the 19 cases of intestinal injury in this series revealed that in 18 both x-rays and radium were used. In the single case which received x-ray therapy alone, conclusive evidence as to whether the injury was the result of irradiation or neoplastic invasion was not available. As in other series previously reported, the use of both x-rays and radium decreased the accuracy of depth dose determination and made the estimation of tolerance doses more difficult. Practically all the previous reports concern the effect of radiation in females treated for carcinoma of the cervix or body of the uterus. The tolerance dose for the normal small and large bowel was not determined within a sufficiently narrow range to be of practical value, as the combined treatment and the possibility of extension of the primary lesion prevented accurate conclusions. During World War II, 280 patients with tumor of the testis were treated at Walter Reed General Hospital, the majority by surgery and roentgen irradiation (2). In the series of cases to be reported here, 1,000-kv. roentgen therapy was used for the treatment or prophylaxis of retroperitoneal metastases, whereas previous authors reporting irradiation damage to the intestines have employed lower voltages, 200 to 400 kv. In this group of young patients there was no pre-existing gastrointestinal disease. This fact, which made it possible to study effects of irradiation on the gastro-intestinal tract uncomplicated by disease processes, the use of roentgen therapy alone, and the relatively large size of the group, afforded a unique opportunity to attempt to determine the tolerance dose of the normal large and small intestine in the adult male. Pathology Clinical evidence of injury to the gastrointestinal tract by irradiation was reported by Walsh (3) in 1897, but the first fatal case with autopsy findings was not published until 1917 (4). The histopathology of irradiation injury of the gastro-intestinal tract, in its experimental and clinical aspects, has been completely reviewed by Mulligan (5) and Warren and Friedman (6).