Objective: To evaluate the prevalence of antibodies to hepatitis C virus (anti-HCV), their relation to outcome, and the seroconversion rate in patients with post-transfusion non-A, non-B hepatitis. Design: Retrospective analysis of prospectively collected serum specimens. Setting: A referral-based university hospital. Patients: Sixty-three consecutive patients who developed non-A, non-B post-transfusion hepatitis after open-heart surgery. All patients had follow-up with serial serum testing and clinical evaluation. The mean ( +/- SD) duration of follow-up after hepatitis onset was 81 +/- 33 months (range, 13 to 132 months). Seventeen patients recovered after acute-phase illness, whereas 46 developed chronic disease which, in 30 cases, was confirmed histologically. Main Results: Of 32 patients tested before transfusion, 1 (3.1%) had anti-HCV. Fifty-nine (93%) patients were anti-HCV positive during acute-phase hepatitis: Patients with "early" seroconversion ( < 15 days after hepatitis onset) did not differ from those with "late" seroconversion ( > 60 days after onset) in epidemiologic, clinical, and biochemical features. The rate of anti-HCV positivity during acute-phase illness was not significantly different among patients who recovered (76%) compared with those who developed chronic disease (95%). At 6 to 12 months, patients whose disease resolved had lower antibody activity than those with progressive disease. Further, during long-term follow-up (1 to 9 years), 53% of patients whose disease resolved but only 6.9% of patients who had progressive disease became anti-HCV negative. Conclusions: Hepatitis C virus is the major cause of post-transfusion hepatitis in Italy. The time to anti-HCV seroconversion varies widely after hepatitis onset and is not significantly associated with acute-phase features or outcome of disease.