We examined the prevalence of enteric protozoan and helminthic infections and the associations between infection and gender, sexual preference, and sexual practices in 180 consecutive patients at a venereal-disease clinic. Of 163 men, 29 were infected with one or more enteric parasites. None of the 17 women had an enteric infection. The prevalence of infections with Entamoeba histolytica or Giardia lamblia (or both) was 21.5 per cent in homosexual men, 6.2 per cent in bisexual men, and 0 in heterosexual men. There were significant associations between oral-anal sex and infection with E. histolytica (P less than 0.01) or with helminths (P less than 0.05). Homosexuality and oral-anal sex were the most important risk factors in E. histolytica, G. lamblia, and helminthic infections. We conclude the "hyperendemic" enteric protozoan infection rates in homosexual men are related to three factors: the original endemic level in the general population; the prevalence of sexual acts that facilitate transmission; and the frequency of exposure to an infected person.