A recently identified RNA virus, hepatitis G virus (HGV), has been investigated for its role in causing non-A-E hepatitis. The frequency and clinical outcome of HGV infection in children was studied. Two hundred apparently healthy children aged 6 mo to 12 y, and 90 children who had undergone open heart surgery in a prospective study for posttransfusion hepatitis were included in this study. The serum samples were tested for HGV RNA by nested reverse transcription-PCR with primers from the 5'-untranslated region. The HGV RNA viremic rate was found to be 1% (2/200) in apparently healthy children, 30% in children after open heart surgery. Among the 90 children, three were HGV-infected before the surgery. Twenty-four (28%) of the remaining 87 children tested positive for HGV RNA within 6 mo after the surgery. Sixty-five percents of these viremic children eventually became persistently infected at 1 y after surgery. No HGV RNA-positive children exhibited elevated alanine aminotransferase levels during the follow-up period. No coinfections of HGV with the hepatitis C virus or hepatitis B virus were found. Patients of younger age appeared more likely to become chronic carriers. Anti-HCV screening did not reduce the prevalence of HGV infection. In conclusion, in children with open heart surgery, the risk of transfusion-transmitted HGV infection and the chronicity rate have been found to be high. Young age is a risk factor of persistent infection.