Chronic hepatitis C is often a progressive liver disease for which there is no satisfactory treatment. We studied the efficacy of recombinant alpha-interferon or gamma-interferon in the treatment of this disease in comparison with a control group. Thirty patients were randomly assigned to three groups. Ten patients received 7.5 MU alpha-interferon/m2 body surface three times weekly for 3 mo, then 5 MU/m2 for 3 mo and 2.5 MU/m2 for 6 mo. Ten patients were treated with gamma-interferon at a dose of 2 MU/m2 for 6 mo and the other 10 served as controls without treatment. The mean serum ALT levels and liver histological findings improved significantly only in the patients treated with alpha-interferon. No changes were observed in patients treated with gamma-interferon or in controls. Five of 10 patients treated with alpha-interferon had complete responses (mean ALT normal during therapy). After treatment ALT returned to pretreatment levels in two of 5 patients. The long-term response rate after alpha-interferon therapy was 30% at 18 mo. We conclude that alpha-interferon is effective in controlling disease activity in a portion of patients with chronic hepatitis C. High doses of alpha-interferon do not appear to add further benefit in the response rate or relapse rate. gamma-Interferon therapy is ineffective.