Mother-to-infant transmission of hepatitis G virus (HGV [or GBV-C]) was studied in sera from 42 mothers at high risk for bloodborne infections and from their 45 infants (3 twin pairs). Seven (17%) of the mothers had HGV RNA in serum by a polymerase chain reaction assay. One of the 8 (12.5%) infants born to HGV-infected mothers became positive for HGV at 3 months of age. He remained HGV-infected throughout the study (42 months), with no signs of liver disease. His twin sister remained HGV-negative despite the presence of serum and salivary HGV in both the mother and the brother. Analysis of HGV sequences from the infected mother and the infected child confirmed a genetic link between the virus of the mother and the infected child. Thus, mother-to-infant transmission of HGV, presumably occurring at partus, may cause persistent HGV viremia.