In an attempt to investigate the incidence and clinical course of type C viral hepatitis among patients with posttransfusion hepatitis, antibodies to hepatitis C virus (anti-HCV) in sera were measured from 42 prospectively followed cardiovascular surgery patients who developed hepatitis after blood transfusions. Of these, 35 (83.3%) had anti-HCV seroconversion during a 6- to 12-month follow-up period. The mean interval between blood transfusion and onset of active anti-HCV seroconversion was approximately 3 months after the first elevation of serum alanine aminotransferase levels (18.1 vs. 6.4 weeks). There was no correlation between fluctuations in serum alanine aminotransferase levels and anti-HCV titers. Of 26 patients with type C posttransfusion hepatitis who were followed greater than 1 year, 20 (76.9%) continued to have abnormal serum alanine aminotransferase levels. The results indicate that HCV is the major agent of posttransfusion hepatitis in Taiwan. Furthermore, it plays an important role in chronic hepatitis among transfused patients.