Eighteen patients receiving Cobalt 60 irradiation for abdominal or pelvic malignancies were assigned at random to eat either a semi-hydrolyzed diet (Flexical: 10 g % casein hydrolsate; 14 g % triglycerides, 20% of which medium chain; 66% disaccharides) or a normal diet. There are no significant differences between these two groups with respect to age and the ratio of ideal to actual caloric intake. The patients in the control group received on the average a total of 3900 rd and those in the Flexical group 4040 rd. Generally, Flexical appeared to have a significant positive effect on body weight. In addition, radiation-induced diarrhea was not a problem in the Flexical group. In the latter group, serum proteins including immunoglobulins remained essentially unchanged during therapy while a moderate but significant fall was observed in all control patients. No significant difference between the two groups was observed with respect to peripheral blood hematocrit, red and white cell counts. However, the drop in blood lymphocytes following irradiation was significantly less in the Flexical group. The mechanisms of radioprotection are discussed. These preliminary data indicate that the nutritional and perhaps the immunological status of cancer patients receiving intensive irradiation can be maintained by dietary measures.