The relative sexual transmission efficiency of hepatitis B virus (HBV) and human immunodeficiency virus type 1 (HIV-1) was investigated by a prospective study of homosexual men in Pittsburgh, Pa, from the Multicenter AIDS Cohort Study. During the 30-month follow-up, 19.8% and 7.8% of the initially seronegative HBV and HIV-1 groups were estimated to seroconvert to HBV and HIV-1, respectively. The significantly higher cumulative HBV seroconversion rate occurred despite a much lower prevalence of hepatitis B carriers (7% were hepatitis B surface antigen positive) compared with HIV-1 carriers (22% were HIV-1 antibody positive). The sexual exposure profile of HBV and HIV-1 seroconverters was similar during the 6 months prior to seroconversion, supporting the link between anal intercourse and acquisition of either infection. However, insertive, not receptive, anal intercourse was the major risk factor identified for HBV seroconversion, suggesting that transurethral exposure is an important mode of transmission. These data suggest that HBV is transmitted 8.6-fold more efficiently than HIV-1 among homosexual men studied and underscore the benefits of both HBV immunization and use of condoms during intercourse to prevent HBV infection.