A novel virus(TTV) was isolated from patients with post-transfusion hepatitis of unknown etiology. We studied the prevalence of TTV in 26 patients with fulminant hepatic failure. Serum samples at admission from seven(27%) of the 26 patients were positive for TTV. TTV was also detected in 29(27%) of the 106 healthy blood donors. There were no differences in the positive rate between patients with fulminant hepatic failure and healthy blood donors. There were no differences in clinical findings, or duration from onset to coma in patients with and without TTV. However, the outcome of the patients with TTV was significant worse than those without. TTV may not cause severe hepatitis such as fulminant hepatic failure.