This study evaluated the magnitude and distribution of unrecognized HIV infection among young men who have sex with men (MSM) and of those with unrecognized infection, the prevalence and correlates of unprotected anal intercourse (UAI), perceived low risk for infection, and delayed HIV testing. MSM aged 15-29 years were approached, interviewed, counseled, and tested for HIV at 263 randomly sampled venues in 6 US cities from 1994-2000. Of 5649 MSM participants, 573 (10%) tested positive for HIV. Of these, 91% of black, 69% of Hispanic, and 60% of white MSM (77% overall) were unaware of their infection. The 439 MSM with unrecognized infection reported a total of 2253 male sex partners in the previous 6 months; 51% had UAI; 59% perceived that they were at low risk for being infected; and 55% had not tested in the previous year. The HIV epidemic among MSM in the United States continues unabated, in part, because many young HIV-infected MSM are unaware of their infection and unknowingly expose their partners to HIV. To advance HIV prevention in the third decade of HIV/AIDS, prevention programs must reduce unrecognized infection among young MSM by increasing the demand for and availability of HIV testing services.