HIV sexual transmission risk behaviors were examined among 1,065 Latino and 1,140 black men who have sex with men (MSM). Participants completed a computer-administered questionnaire and were tested for HIV infection. Of men who reported that their last HIV test was negative or that they had never been tested or did not get the result of their last test, 17% of black and 5% of Latino MSM tested HIV-positive in our study. In both ethnic groups, the three-month prevalence of unprotected anal intercourse (UAI) with HIV-negative or unknown serostatus partners was twice as high among men unaware of their HIV infection than men who knew they were HIV seropositive at the time of enrollment. UAI exclusively with HIV-positive partners was more prevalent among HIV-positive/aware than HIV-positive/unaware men. The findings advance understanding of the high incidence of HIV infection among black MSM in the U.S.