The efficacy of interferon therapy (IFN) was investigated in 46 patients with chronic non-A, non-B (NANB) hepatitis, of would 40 (87.0%) were positive for anti-HCV antibody (Ab) (C-100-3). Three kinds of IFN were used; human lymphoblastoid interferon (HLBI), interferon alpha-2b and interferon beta. Total doses of IFN ranged from 1 million units (MU) to 10 MU and treatment duration ranged from 2 weeks to 144 weeks. Of 46 patients 34 (73.9%) responded to IFN. Nine patients have maintained normal ALT levels and 5 patients have maintained near-normal ALT levels for more than 6 months after cessation of IFN treatment. In these cases the titers of anti-HCV Ab had decreased significantly at the end of IFN therapy and 6 months after IFN therapy respectively. The mean age was young and the mean disease duration was short in effective cases. As for doses and treatment duration of IFN, low doses of IFN requires long treatment duration to acquire continuous efficacy and high doses of IFN requires rather short treatment durations. Therefore, early IFN treatment, higher doses and longer periods of IFN treatment may improve the response rate of patients with chronic NANB hepatitis.