Introduction: Extensive observational data suggest that herpes simplex virus type 2 (HSV-2) infection may facilitate HIV acquisition, increase HIV viral load, and accelerate HIV progression and onward transmission. To explore these relationships, we examined the impact of preexisting HSV-2 infection in an international HIV vaccine trial. Methods: We analyzed the associations between prevalent HSV-2 infection and HIV-1 acquisition and progression among 1836 men who have sex with men. We used Cox proportional hazards regression models to estimate the association between HSV-2 infection and both HIV acquisition and antiretroviral therapy (ART) initiation, and linear regression to explore the effect of HSV-2 on pre-ART viral load. Results: HSV-2 infection increased risk of HIV-1 acquisition among all volunteers [adjusted hazard ratio 2.2; 95% confidence interval (CI): 1.4 to 3.5]. Adjusting for demographic variables, circumcision, Ad5 titer, and significant risk behaviors, the risk of HIV acquisition among HSV-2-infected placebo recipients was 3-fold higher than HSV-2 seronegatives (adjusted hazard ratio 3.3; 95% CI: 1.6 to 6.9). Past HSV-2 infection was associated with a 0.2 log10 copies per milliliter higher adjusted mean set point viral load (95% CI: 0.3 lower to 0.6 higher). HSV-2 infection was not associated with time to ART initiation. Conclusions: Among men who have sex with men in an HIV-1 vaccine trial, preexisting HSV-2 infection was a major risk factor for HIV acquisition. Past HSV-2 did not significantly increase HIV viral load or early disease progression. HSV-2-seropositive persons will likely prove more difficult than HSV-2-seronegative persons to protect against HIV infection using vaccines or other prevention strategies. © 2011 Lippincott Williams & Wilkins.