The purpose of this study was to characterize the risk of human immunodeficiency virus (HIV) infection for men who have sex with men and to identify the risk such men pose to their female sex partners. The subjects were 5480 men who were tested for HIV between January 1987 and December 1991 and who reported having had sex with a man since 1977. Men who identified themselves as bisexual or straight were more likely to use injection drugs, had a substantial HIV seroprevalence, and reported many more female partners than men who identified themselves as gay. Men who identify themselves as bisexual pose the greatest risk to their female partners.