The present analysis was designed to determine whether race/ethnicity was independently associated with mother-to-child HIV-1 transmission risk in subjects enrolled in a trial of 2-dose intra-partum nevirapine in combination with standard antiretroviral therapy and to determine what factors, including race/ethnicity, predicted maternal viral suppression at the time of delivery. Women enrolled in Pediatric AIDS Clinical Trials Group (PACTG) 316 from sites in the United States and Puerto Rico were included. Distribution of selected maternal disease and treatment characteristics was assessed by race/ethnicity category. Logistic regression models were fit to evaluate possible association of factors with HIV transmission and with viral load at delivery. Variables associated with the outcome at P < 0.05 level were retained in the final models. Of 1052 women randomized at PACTG sites, 891 were included in the present analysis: 572 (64%) were black; 206 (23%) were Hispanic; and 113 (13%) were white. All women who had infected infants were black or Hispanic (11/572 and 3/206, respectively), whereas none of the women identified as white had an infected infant (0/113). This difference was not statistically significant (P = 0.54). White women had higher entry CD4 cell counts and lower HIV-1 RNA at delivery than women of other races/ethnicities. Black and Hispanic women were more likely than white women to start therapy during their current pregnancy but did not initiate prenatal care later. In bivariate models that included antiretroviral type and variables that had values of P < or = 0.25 in univariate analysis, time of antiretroviral initiation, time of prenatal care initiation, and race/ethnicity each retained significance in predicting viral suppression at delivery. Race/ethnicity remained predictive of viral suppression at delivery in a multivariate model incorporating all of these variables (P = 0.01). Higher HIV-1 RNA and lower CD4 cell counts in women identified as black or Hispanic have significant implications for the health of these women and their newborns. Race/ethnicity is significant in predicting viral suppression at the time of delivery.