To determine the contribution of herpes simplex type 2 (HSV-2) infection to the risk of human immunodeficiency virus (HIV) acquisition, a systematic review of literature and data synthesis were done. Thirty-one studies addressed the risk of HIV infection in HSV-2-seropositive persons. For 9 cohort and nested case-control studies that documented HSV-2 infection before HIV acquisition, the risk estimate was 2.1 (95% confidence interval, 1.4-3.2). Thus, the attributable risk percentage of HIV to HSV-2 was 52%, and the population attributable risk percentage was 19% in populations with 22% HSV-2 prevalence but increased to 47% in populations with 80% HSV-2 prevalence. For 22 case-control and cross-sectional studies, the risk estimate was 3.9 (95% confidence interval, 3.1-5.1), but the temporal sequence of the 2 infections cannot be documented. Control strategies for HSV-2 need to be incorporated into control of sexually transmitted infections as a strategy for HIV prevention.