The risk of human immunodeficiency virus (HIV) transmission from various types of homosexual contact, including oral sex, is of biologic, epidemiologic, and public health importance. The per-contact risk of acquiring HIV infection from specific acts was estimated in a prospective cohort study of 2,189 high-risk homosexual and bisexual men, conducted in San Francisco, California; Denver, Colorado; and Chicago, Illinois, in 1992-1994. During 2,633 person-years of follow-up, 60 seroconversions were observed. The estimated per-contact risk of acquiring HIV from unprotected receptive anal intercourse (URA) was 0.82 percent (95% confidence interval: 0.24, 2.76 percent) when the partner was known to be HIV+ and 0.27 percent (95% confidence interval: 0.06, 0.49 percent) when partners of unknown serostatus were included. There was heterogeneity in per-contact risk, with nine seroconversions occurring after only one or two episodes of URA. The per-contact risk associated with unprotected insertive anal and receptive oral sex with HIV-positive or unknown serostatus partners was 0.06 and 0.04 percent, respectively. URA accounted for only 15 percent of all reported sexual activity by seroconverters. As lower-risk practices become more common, they may play a larger role in propagating the epidemic and should also be addressed by interventions targeting high-risk homosexual and bisexual men.