OBJECTIVE To assess the risk of perinatal human immunodeficiency virus (HIV) transmission in HIV-infected women clinically diagnosed with genital herpes simplex virus (HSV) infection during pregnancy. METHODS This retrospective analysis included 402 HIV-infected pregnant women who enrolled from 1994-1999 in a multicenter prospective cohort study in New York City, who delivered a liveborn singleton infant with known HIV infection status, and who had information on diagnosis of genital HSV infection during pregnancy. Study participants were determined to have genital HSV infection during pregnancy by documentation of clinical diagnosis. RESULTS Forty-six (11.4%) of the study participants delivered HIV-infected infants. Twenty-one (5.2%) had clinical diagnosis of genital HSV infection in pregnancy. Six (28.6%) of the 21 HIV-infected women with a clinical diagnosis of genital HSV infection delivered an HIV-infected infant. In univariate analyses, HIV-infected pregnant women with clinical diagnosis of genital HSV infection during pregnancy had a significantly increased risk of perinatal HIV transmission (odds ratio 3.4, 95% confidence interval 1.3-9.3; P = .02). When other factors associated with perinatal HIV transmission were included in a logistic regression model (lack of zidovudine therapy during pregnancy or delivery, prolonged rupture of membranes, and preterm delivery), clinical diagnosis of genital HSV infection during pregnancy remained a significant independent predictor of perinatal HIV transmission (adjusted odds ratio 4.8, 95% confidence interval 1.3-17.0; P = .02). CONCLUSIONS Clinical diagnosis of genital HSV infection during pregnancy in HIV-infected women may be a risk factor for perinatal HIV transmission. If future studies confirm this association, therapy to suppress genital HSV reactivation during pregnancy may be a strategy to reduce perinatal HIV transmission.