Hepatitis C virus (HCV) carriers with normal aminotransferase levels often show histological chronic hepatitis. This study was undertaken to determine the effect of interferon (IFN) in such patients. Nineteen HCV carriers with normal aminotransferase activities and chronic hepatitis were randomized to receive IFN-alpha2b (3 million units 3 times weekly for 12 months) or no treatment. Therapy was monitored by qualitative and quantitative determination of viral RNA. Patients who did not clear HCV RNA after 6 months discontinued therapy. In all, 9 patients constituted the control group, while 10 patients were treated. Five of these patients, still viremic after 6 months, stopped IFN. The remaining 5 patients, who cleared the viral RNA within 6 months, completed the 12-month course. Three of these patients relapsed off treatment, and 2 were still free of viremia 12 months after stopping therapy. A transient flare-up of aminotransferase activities was detected in 2 patients during treatment and in 3 patients after. None of the 9 control patients cleared the viral RNA during follow-up. A variable degree of sequence heterogeneity was detected in the hypervariable region before therapy, and IFN treatment decreased sequence diversity in all patients. These results indicate that IFN therapy can be effective in chronic HCV carriers with normal aminotransferase activities, inducing short-term virological response in 3 of 10 patients and sustained response in 2. The effects of treatment on viral load and quasispecies complexity were similar to those reported previously in patients with increased aminotransferase activities.