A multitude of opportunistic infections has been documented in virtually every organ system of patients with the acquired immunodeficiency syndrome (AIDS). Prominent among these are infections of the gastrointestinal tract. However, studies of large numbers of patients documenting the frequency of such involvement are lacking. We reviewed the records of 100 patients with AIDS and assessed the frequency and organisms causing infectious diarrhea. We found diarrhea to be more common in homosexuals (80%) than heterosexuals with a risk factor of parenteral drug use (58%). In one third of all cases, no infectious etiology was found. Myobacterium avium intracellulare (MAI) was the most commonly identified cause of infectious diarrhea in our series, followed by cytomegalovirus, cryptosporidium, Salmonellaspp., and herpes virus. In addition, bacteremia was documented in 43% of patients with infectious diarrhea and was most commonly due to MAI. Finally, we demonstrated that multiple concurrent infections are not uncommon (22%) in AIDS patients and that the diarrheal syndrome may not respond unless all pathogens are eradicated.