A cross-sectional human immunodeficiency virus type 1 (HIV−1) serosurvey was conducted between January 1988 and January 1993 at a New York City sexually transmitted disease clinic serving predominantly African-American and Hispanic patients. Overall, 14% (415/3, 069) of participating men reported having sex with men; among these, only 52 (13%) were classified as “homosexual” (having had sex with men exclusively since 1978). Most men (87%) who reported having sex with other men also reported having sex with women. These included 147 (35%) “bisexuals” (sex with more than one man and at least one woman since 1978) and 216 (52%) “heterosexuals” (sex with women since 1978 and sexual contact with men before 1978 or only once thereafter). Although HIV−1 seroprevalence was highest among “homosexual” men (70%, 95% confidence interval (Cl) 55−81), it was also high among “bisexual” men (35%, 95% Cl 27−43) and “heterosexual” men (17%, 95% Cl 13−23), and was lowest in men who reported having no male sex partners in their lifetime (9%, 95% Cl 7−10). It is possible that transmission of HIV−1 from bisexual men to female sexual partners plays a greater role in heterosexual transmission than was previously recognized. Am J Epidemiol 1998; 147: 269−72.