OUTLINE: This is a randomized, double-blind, placebo controlled study. Patients are stratified by history of anterior resection of the rectum (yes vs no); total planned cumulative dose, including boost fields of external beam radiotherapy (4500-5350 cGy vs 5350-6000 cGy vs greater than 6000 cGy); use of fluorouracil (none vs bolus vs continuous infusion); and primary site (rectal cancer vs prostate cancer vs gynecological cancer vs other). Beginning the first or second day of radiotherapy, patients receive either oral glutamine or a placebo twice daily, including the days that they do not receive radiotherapy. Patients continue on treatment throughout radiotherapy and continue 2 weeks postradiotherapy or until grade 3 diarrhea occurs. Patients are followed weekly for 4 weeks, then at 12 months, and then at 24 months after radiotherapy.