A randomized, controlled trial was undertaken to evaluate the response to recombinant interferon alfa-2a in patients with hepatitis B surface antigen negative chronic active hepatitis of unknown aetiology (community-acquired non-A, non-B hepatitis). Thirty patients were treated with thrice weekly interferon in a dose of 6 million units for 1 month, followed by 3 million units for 3 months and 1 million units maintenance therapy for 8 months. Patients who relapsed were returned to the previously effective dose. A control group of 30 patients received no treatment and were followed up for 12 months. Overall, 21/30 (70%) of treated patients had a complete response, with normalization of serum aminotransferase levels, and a further three (10%) had a partial response (a decrease to less than 50% of baseline levels). No significant changes were observed in the control patients. Five (24%) of the 21 complete responders relapsed during months 2-4 and 8/21 (38%) relapsed during maintenance therapy. Three (23%) of these 13 patients had a return to normal serum aminotransferase levels when the dose was increased and 7/13 (54%) demonstrated a significant decrease. In a retrospective analysis of patients' sera for antibody to hepatitis C virus, 23/26 patients testing positive were treatment responders compared to 1/4 antibody negative patients. Our results suggest that interferon alfa-2a is an effective treatment for community-acquired non-A, non-B hepatitis, but reactivation during treatment occurred in 62% of patients. We recommend use of a higher dose of interferon for at least 1 year with long-term follow up in order to exclude early or late reactivation after treatment withdrawal.