The relation between herpes simplex virus type 2 (HSV-2) and human immunodeficiency virus (HIV) acquisition was evaluated among 4,295 high-risk, HIV-negative men who have sex with men in an intensive behavioral intervention (colloquially referred to as "EXPLORE") study in the United States from 1999 to 2003. Sexual behavior data were obtained by computer-assisted self-interview, and sera were collected semiannually for HIV and HSV-2 serology. HSV-2 infection was classified as "recent incident" (at the first HSV-2 seropositive visit), "remote incident" (within 24 months of the first positive visit), and "prevalent" (for visits >24 months after the first HSV-2 positive visit). Baseline HSV-2 prevalence was 20.3%. HSV-2 incidence was 1.9 (95% confidence interval (CI): 1.6, 2.2) per 100 person-years; significant risk factors were African-American race, unprotected receptive anal intercourse, an HIV-positive male sex partner, and six or more male partners in the prior 6 months. The behavioral intervention did not reduce HSV-2 acquisition (adjusted hazard ratio (HR) = 1.2, 95% CI: 0.9, 1.6). Overall HIV incidence was 1.9 (95% CI: 1.7, 2.2) per 100 person-years. HIV risk was elevated among men who have sex with men with recent incident HSV-2 (adjusted HR = 3.6, 95% CI: 1.7, 7.8), remote incident HSV-2 (adjusted HR = 1.7, 95% CI: 0.8, 3.3), and prevalent HSV-2 (adjusted HR = 1.5, 95% CI: 1.1, 2.1) infection compared with HSV-2 seronegative participants. HIV intervention strategies targeting HSV-2 prevention and suppression among men who have sex with men should be evaluated.