The risk of human immunodeficiency virus (HIV) transmission from various types of homosexuel 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 homosexuasl and bisexual men, conducted in San Fracisco, California; Dener, Colorado; and Chivcago, Illinosl, 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 arial intercouse (URA) was 0.82 percnt (95% confidecne Interval: 0.24, 2, 76 percnt) when the partner was known to be HIV+and 0.27 percnt (95% confidence interval: 0.06, 0.49 percnt) when partners of unknown serostatus were included. There was heterogenlity In per confact risk associated with unportectd ininsertive 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 reproted sexual activity by seroconverters. As lower-risk practives become more common, they may play a larger role in propagating the epidemic anc should also be addressed by interventions targeting high-risk homosexual and bisexual men. Am J. Epidemiol 1999; 150; 306–11.