We reanalyzed the results of a pilot study of recombinant alpha-interferon therapy for chronic non-A, non-B hepatitis in light of the recent discovery of the hepatitis C virus and the development of diagnostic assays for this agent. Stored serum samples from 10 patients treated between 1984 and 1986 were tested for antibody to hepatitis C virus and hepatitis C virus RNA before, during and after therapy. In addition, the current clinical, serum biochemical and virological statuses of these patients were evaluated to determine the long-term effects of interferon therapy. All patients had evidence of hepatitis C virus infection, with hepatitis C viral RNA, antibody to hepatitis C virus or both markers detectable in serum. Serum hepatitis C virus RNA was found to disappear in seven of eight patients whose aminotransferase levels became normal with interferon therapy but remained present in two patients who did not respond to therapy. Levels of hepatitis C virus RNA decreased and disappeared when serum aminotransferases fell to normal levels but rose with subsequent elevation of aminotransferase levels in two patients who had relapses in disease when interferon was stopped. During a follow-up of 3 to 6 yr, hepatitis C virus RNA remained undetectable in the six patients whose serum aminotransferase levels remained normal after interferon therapy. However, neither initial titers of hepatitis C virus RNA nor disappearance of viral RNA from serum during treatment predicted a sustained response. Thus long-term beneficial responses to alpha-interferon can occur in patients with chronic hepatitis C and are associated with sustained loss of hepatitis C virus RNA from serum.