In an attempt to investigate the incidence and clinical course of non-A, non-B (NANB) hepatitis following blood transfusion in Taiwan, 288 patients who underwent cardiovascular surgery and received blood transfusion were followed prospectively with serum liver aminotransferase levels and viral hepatitis markers for at least six months. None had any past history of liver disease or drug abuse. All blood donors were tested for serum hepatitis B surface antigen and alanine aminotransferase (ALT) (greater than 45 U/L). Thirty-seven (12.8%) patients developed PTH. 34 (91.9%) were considered to be cases of NANB hepatitis, 2 (5.4%) were cytomegalovirus hepatitis, and one (2.7%) was caused by Epstein-Barr virus. No one developed hepatitis B post-transfusion hepatitis (PTH). Of the 34 NANB PTH patients, 15 (44.1%) were asymptomatic, 16 (47.1%) had clinical symptoms, and 9 (26.5%) had serum total bilirubin levels higher than 2 mg/dl. There was no case of fulminant hepatic failure. Of 26 NANB PTH patients who were followed up for more than one year, 15 (57.7%) still had abnormal serum ALT levels. The incubation period of NANB PTH ranged from 2 to 16 (mean 6.1 +/- 3.2) weeks. Of the 37 PTH patients, 32 (86.5%) were found to have anti-HCV seroconversion during one year follow-up period. NANB PTH is as common in Taiwan as in the United States and Japan, and is demonstrated by this study to be due mostly to HCV.