Hepatitis due to hepatitis delta virus (HDV) infection is generally associated with severe histological abnormalities and rapid progression of the disease. To assess the efficacy of recombinant interferon-a2b in treatment of chronic delta hepatitis, 22 patients were entered into a randomized controlled trial: 11 received interferon-a2b subcutaneously three times weekly for 12 months (5 MU/m2 for 4 months and then 3 MU/m2 for a further 8 months) and 11 were untreated. All patients were followed up for 6 months after the completion of therapy. Nine treated patients completed the trial: one was withdrawn with hyperthyroidism and one committed suicide. Serum ALT levels were normalized or significantly reduced, always within 3 months of initiating treatment, and remained so in 73% of treated patients at the 4th month and in 54.5% at the 12th month, compared with 18% and 18%, respectively, in the untreated group. Moreover, in seven of nine treated patients, interferon was associated with the clearance of serum HDV-RNA, associated with amelioration of the histological picture, whereas this occurred in only four of 11 untreated patients. On cessation of therapy, all patients but one experienced a biological and/or virological relapse over the 6-month follow up. In conclusion, our data confirm that HDV is sensitive to inhibition by interferon-a2b, although the schedule used did not achieve permanent control of the disease. The adverse effects of interferon require consideration; in particular, care will be needed to avoid serious psychiatric side effects.