Since December 1989, the Japan Red Cross Blood Bureau has screened blood donors for hepatitis C virus antibody on the basis of enzyme-linked immunosorbent assay and high titers of antibody to HBc antigen. To elucidate the effectiveness of the new screening tests in preventing posttransfusion hepatitis, the incidence of posttransfusion hepatitis after the introduction of the new tests (December 1989 to December 1990) was compared with the incidence before their introduction (January 1982 to December 1988). The incidence of posttransfusion hepatitis was 9.8% (219 of 2240), with a mean transfusion volume of 10.2 units, before the screening and 3.7% (12 of 326), with a mean transfusion volume of 14.7 units, after the introduction of the new tests. Statistical analysis revealed a significant decrease of incidence of posttransfusion non-A, non-B hepatitis after the introduction of the new tests (X2 = 10.9, p < 0.01). Posttransfusion hepatitis B occurred in 3 of 2,240 recipients (0.13%) before the introduction of HBc antibody testing. No cases of posttransfusion B viral hepatitis developed after the introduction of the new tests. Hepatitis C virus antibody status was investigated in 7 of 12 posttransfusion hepatitis patients who contracted the disease after the new screening tests were initiated. Hepatitis C virus antibody seroconversion occurred in three of the seven cases (43%), as detected on first- and second-generation hepatitis C virus antibody assays and reverse-transcription polymerase chain reaction.