Using an enzyme-linked immunosorbant assay for antibody against hepatitis C virus (anti-HCV), serial serum samples from 26 non-A, non-B (NANB) posttransfusion hepatitis (PTH) patients were studied in a prospective study in Taiwan. Sixteen (61.5%) of the 26 patients were positive for anti-HCV antibodies. Two of the 16 patients were positive for anti-HCV before transfusion. The remaining 10 patients were negative for anti-HCV antibodies. The rate of anti-HCV seroconversion is, therefore, 58.5%. Of the 14 patients with anti-HCV seroconversion, three were hepatitis B surface antigen (HBsAg) carriers. The time of seroconversion for anti-HCV ranges from 2 to 24 weeks after the first elevation of ALT (mean: of 8.7 weeks,) or 6-32 weeks from the date of transfusion (mean: 13 weeks). Twelve (85.7%) of the 14 anti-HCV seroconverted patients had persistent abnormal ALT 6 months after the onset of hepatitis in contrast to 30% of chronicity in the anti-HCV-negative patients. The results suggest that HCV is the major causative agent in NANB PTH in Taiwan, and patients positive for anti-HCV have a higher risk of chronicity.