The efficacy and safety of ganciclovir therapy for cytomegalovirus (CMV) colitis in patients with AIDS was examined in a double-blind, placebo-controlled study. Sixty-two patients at four university medical centers were enrolled. All had biopsy-proven CMV colitis with diarrhea, fever, and weight loss. Other pathogens were excluded. Ganciclovir (5 mg/kg) or placebo was administered every 12 h for 14 days. A significant reduction in CMV-positive colonic and urine cultures was seen with ganciclovir (P = .034 and P < .001, respectively) compared with placebo. Colonoscopy scores were improved significantly more with ganciclovir than with placebo (P = .042). New extracolonic CMV disease developed in 7 (23%) of 30 placebo patients and in 3 (9%) of 32 ganciclovir patients in only 14 days (P = .026). Ganciclovir-treated patients maintained body weight, while placebo patients had a mean loss of 1.5 kg. Overall, ganciclovir appears of some benefit in treating CMV colitis in patients with AIDS.