Isospora belli is an uncommon cause of diarrhea in man. Like the closely related Cryptosporidium, the organism causes disease that may be more severe and chronic in patients with underlying immune deficiency states. We describe three male homosexuals with Isospora enteritis. Each had several months of diarrhea. All three patients had lymphopenia and two had profound T cell helper-suppressor ratio reversal. In addition, two of the patients admitted to oral-anal contact, a practice associated with the acquisition of parasites and other enteric pathogens in the homosexual population. We suggest that Isospora belli may be a sexually transmitted pathogen which should be considered in the diagnosis of homosexual patients with diarrhea.