The sexual behaviors of bisexually active men, defined as men having sex with a man and a woman in previous 6 months, were compared with men who had sex with men only. Differential sexual practices associated with HIV risk between the two groups of men, as well as in the bisexual men with their male and female partners, were evaluated. Cross-sectional analyses were performed on baseline data from a prospective cohort of 508 young gay men recruited from bars, college campuses, and a health center in Boston from 1993 to 1994. Odds ratios (OR) and 95% confidence intervals (CI) were calculated on categorical variables, and McNemar's X2 was used to compare the behaviors of bisexual men with their male versus female sex partners. Six months before the interview, 47 (10%) men had male and female sex partners, and 383 men had only male sex partners during the past year or ever. Fifty-eight percent of the men in the study had a female sexual partner in their lifetime, and 18% during the past year. Bisexual men were more likely to have drinking problems as identified by the Michigan Alcoholism Screening Test (MAST; OR = 3.96, 95% CI = 1.54-10.20), and fewer male partners over their lifetime (mean ± standard deviation [SD], 24 ± 42; median, 7; versus mean ± SD, 69 ± 516; median, 12), although this difference was not statistically significant. The two groups had similar levels of unprotected anal intercourse (25.5% versus 29.5%); however, bisexual men were half as likely to have anal sex as homosexual men (OR = 0.50; 95% CI = 0.27-0.93). Bisexual men were three times as likely to have unprotected sex with their female partner as their male partner (OR = 3.0; 95% CI = 1.02-8.8). Stratified analysis revealed similar discordant behavior while sober (OR = 4.0), drinking (OR = 7.0), and while drinking with concurrent drug use (OR = 8.0). Among this cohort of men who have sex with men (MSM), a sizable proportion also had vaginal sex with female partners in the previous 6 months. Bisexually active men were more likely to have unprotected sex with their female partners compared with their male partners, potentially increasing the risk for HIV and other sexually transmitted diseases. Behavioral interventions directed toward MSM need to address bisexual behaviors.