Hepatitis C virus (HCV) infection was investigated retrospectively and prospectively in children who had open heart surgery. The retrospective study included 196 patients who had open heart surgery 2 to 6 years before enrollment and were regularly followed. Antibody to HCV was detected in eight cases, and seven of these eight patients had viremia caused by HCV infection. The other patient's anti-HCV titer gradually decreased and HCV ribonucleic acid was absent throughout the course. Four of these eight had persistent viral replication without clinical evidence of hepatitis; the other three had viremia and chronic hepatitis. The HCV infection rate in the retrospective study group was about 4%. The HCV was of genotype 2 in five cases and of genotype 3 in two cases. The clinical outcome was not related to sex, age, amount of transfusion, or HCV genotype. For the prospective study, we enrolled 94 patients. Of the 56 enrolled after the initiation of anti-HCV screening in the blood bank, none was infected by HCV. Of the 94 patients, 38 were enrolled before screening; 4 had abnormal liver function 1 to 3 months after operation, and 2 were infected with HCV. One patient had an acute but resolving course; the other had persistent anti-HCV and HCV viremia, although the liver function test results returned to the normal range. The HCV in both cases belonged to genotype 2. We conclude that in Taiwan the seroconversion rate of HCV in children who underwent open heart surgery was 4% to 5%. Anti-HCV screening in donor blood significantly decreased the incidence of posttransfusion hepatitis C.