Interviews were conducted with 750 men, recruited from a variety of sources in Chicago, who reported sex with men in the past 3 years. Behavioral criteria were used to establish groups of gay and bisexual men. We predicted that gay men, compared to bisexual men, would report more male sexual partners, more experience with receptive sex, and more tolerant attitudes toward homosexuality. The only reliable difference between the gay and bisexual men with respect to number of partners was that gay men were more likely to have had a steady male partner or lover. Gay men were more likely than bisexual men to have engaged in receptive sex, including unprotected receptive anal sex. Bisexual men were more self-homophobic and saw other people as less accepting of same-sex activity. There were no differences between gay and bisexual men in other psychosocial variables. Interventions designed to reduce the transmission of HIV/AIDS need to consider differences in gay and bisexual men's sexual behavior and attitudes toward homosexual behavior.