Gastrointestinal viruses, predominantly rotaviruses and adenoviruses, were detected by enzyme-linked immunosorbent assay, electron microscopy, or cell culture in greater than 50% of two groups of homosexual men with symptomatic human immunodeficiency virus (HIV) infection, who did (54%) or did not (50%) have diarrhea. Lower detection rates were observed in HIV-seronegative (15%) and asymptomatic HIV-seropositive (16%) men. In the patients with diarrhea, 95% of the isolates of virus were found in the most immunosuppressed patients, those patients with AIDS-related complex or opportunistic infections associated with AIDS. High excretion rates of these viruses are probably associated with both anal-oral transmission and immunosuppression. These viruses apparently cause acute episodes or relapses of diarrhea in some patients but may be co-pathogens or noncontributory to chronic diarrhea in others.