To estimate the risk of mother-to-child transmission of hepatitis C virus (HCV) and identify correlates of transmission, 245 perinatally exposed singleton children followed prospectively beyond 18 months of age were studied. Overall, 28 (11.4%) of the 245 children acquired HCV infection. Transmission occurred in 3 of 80 children (3.7%) whose mothers had HCV infection alone and in 25 of 165 (15.1%; P < .01) whose mothers had concurrent infection with human immunodeficiency virus type 1 (HIV-1). The percentage of HIV-1-infected children was similar (22 of 165, 13.3%), but each virus was transmitted independently; only six infants (3.6%) were coinfected with HCV and HIV-1. The risk of HCV transmission was not associated with maternal HIV-1-related symptoms, intravenous drug use, prematurity, low birth weight, or breast-feeding, whereas it was lower with cesarean section than with vaginal delivery (5.6% vs. 13.9%, P = .06). This suggests that transmission occurs mainly around the time of delivery.