As part of a double-blind, placebo-controlled study of ganciclovir in cytomegalovirus (CMV) colitis, the clinical characteristics of 44 patients enrolled at one center were analyzed in detail. All were homosexual men who had CMV on colonic biopsy. CMV colitis was the index diagnosis for acquired immune deficiency syndrome (AIDS) in 11 (25%) of the 44 patients. All had diarrhea, but it was intermittent in 13 patients (30%). Bleeding was uncommon, but 35 patients (80%) were febrile (median temperature of 38.9 degrees C). Weight loss was reported by 39 patients (89%), among whom the median loss was 6.8 kg. Endoscopy revealed normal colonic mucosa but CMV on biopsy in 11 patients (25%). Colonoscopic biopsies positive for CMV were found only in the cecum in 7 (39%) of 18 patients. Most patients (54%) had received zidovudine before the diagnosis of CMV colitis. The median time to the development of CMV colitis after the diagnosis of AIDS was 16 months in those patients who had received zidovudine and 3 months in those who had not (p less than 0.02). We conclude that CMV colitis can present early in AIDS and often with such nonspecific signs as fever, intermittent diarrhea, weight loss, and hematochezia. Importantly, it can appear normal on colonoscopy and occurs frequently only in the right colon, necessitating full colonoscopy and multiple biopsies for accurate diagnosis.