The rate of vertical transmission of hepatitis C virus (HCV) was determined by a combination of assays for anti-HCV antibody and the polymerase chain reaction (PCR) in 66 children born to infected mothers. Only 4 children showed evidence of infection with HCV, being positive for anti-HCV in all samples collected from 6 months to 5 years of age. All samples from the remaining 62 children were repeatedly anti-HCV-negative on screening by two second-generation antibody assays. Furthermore, samples collected at age 12 months from 30 antibody-negative children born of HCV-infected mothers were uniformly PCR-negative, showing that “seronegative” infection with HCV was rare or absent in this study group. Serologic reactivity to HCV-encoded antigens in samples from infected children was largely confined to the HCV core protein. Infection with human immunodeficiency virus in the mother was not a significant cofactor for mother-to- child transmission of HCV.