A prospective randomized trial to determine the value of a low fat diet with or without cholestyramine in the treatment of acute intestinal complications of pelvic irradiation is presented. A total of 35 patients receiving pelvic irradiation were entered in the study and all patients had received a 40 gm fat diet. The group was then randomized to receive either placebo (17 patients) or cholestyramine (18 patients). Diarrhea occurred in six out of 16 evaluable patients in the control group and only one of the 17 evaluable patients in the cholestyramine group. The frequency of diarrhea and the diarrhea scale remained high in the placebo group in the entire observation period. Statistical analysis had revealed better diarrhea control in the cholestyramine group (p = less than 0.05). In this report mechanism by which diarrhea occurs following pelvic irradiation is discussed. The adverse effects associated with the use of cholestyramine have been presented. It was concluded that cholestyramine is effective in preventing acute diarrhea induced by pelvic irradiation in patients receiving a low fat diet but is associated with side effects.