We evaluated vertical transmission of hepatitis C virus (HCV) in 37 pregnant women, 20 of whom also had human immunodeficiency virus (HIV) antibody. The HCV subtypes 1a and 3a were prevalent among pregnant women with HIV infection. Infection with HCV was transmitted from 30.7% of the 13 mothers with HCV ribonucleic acid (RNA) and HIV antibody and from 25% of the 8 with HCV RNA alone. No mother with HCV antibody but no HCV RNA transmitted HCV to her infant. Subtypes 1b and 3a seemed to be the most common HCV genotypes transmitted.