Among homosexual men the prevalence of infection with Entamoeba histolytica is high. To determine the clinical importance of this infection 55 homosexual men carrying the parasite were investigated in detail. No clinical, serological, or histological evidence of invasive amoebiasis was found in any of them. The patients were not treated and were followed up for 12 to 29 months (mean 21.6 months), during which period none developed symptoms that could be attributed to E histolytica. Spontaneous loss of the parasite occurred in 17 patients, some of whom later became reinfected. Sixteen patients had antibody to human immunodeficiency virus, and infection with E histolytica showed the same benign course in them as in the patients who did not have antibody. Throughout the study classification of the isolates of E histolytica consistently showed that they belonged only to non-pathogenic zymodemes. The findings provide further evidence that E histolytica in homosexual men is a commensal organism.