Men who have sex with men (MSM) are at increased risk for infection with human immunodeficiency virus (HIV). In 2006, 57% of new HIV infections in the United States occurred among MSM. To estimate and monitor risk behaviors, CDC's National HIV Behavioral Surveillance system (NHBS) collects data from metropolitan statistical areas (MSAs) using an anonymous cross-sectional interview of men at venues where MSM congregate, such as bars, clubs, and social organizations. This report summarizes NHBS data from 2008, which indicated that, of 8,153 MSM interviewed and tested in the 21 MSAs participating in NHBS that year, HIV prevalence was 19%, with non-Hispanic blacks having the highest prevalence (28%), followed by Hispanics (18%), non-Hispanic whites (16%), and persons who were multiracial or of other race (17%). Of those who were infected, 44% were unaware of their infection. Men who know their current HIV infection status can be linked to appropriate medical care and prevention services. Once linked to prevention services, men can learn ways to avoid transmitting the virus to others. Young MSM (aged 18--29 years) (63%) and minority MSM (other than non-Hispanic white) (54%) were more likely to be unaware of their HIV infection. Efforts to ensure at least annual HIV testing for MSM should be strengthened, and HIV testing and prevention programs should increase their efforts to reach young and minority MSM.