Entamoeba histolytica is considered to be an uncommon, imported organism in the United Kingdom and in many parts of North America, but recent attention has been drawn to the possibility of sexual transmission of this parasite among homosexual men. To determine the prevalence and clinical importance of enteric parasitic infections in men attending a clinic in London for the treatment of sexually transmitted diseases, we studied 354 randomly selected patients who provided a single stool sample that was examined for E. histolytica and other intestinal parasites. Forty-five of the 225 homosexual patients (20 percent) were infected with E. histolytica, but no such infections were found among the 129 heterosexual subjects (P less than 0.0001). With the use of isoenzyme electrophoresis, 34 of the 45 E. histolytica isolates were classified according to zymodeme. All were Zymodeme I or III, which are considered to be nonpathogenic. There was no correlation between the presence of E. histolytica and gastrointestinal symptoms. These findings suggest that E. histolytica is a common commensal in the homosexual population and that, in the absence of evidence of invasive disease, treatment of persons passing cysts of the organism may have little practical benefit.