To determine whether long-term therapy with recombinant interferon-alpha can improve the course of chronic delta hepatitis, 61 Italian patients with this disease were randomly assigned to receive either interferon-alpha-2b three times a week (5 MU/m2 for 4 mo and then 3 MU/m2 for another 8 mo) or no treatment. At the end of the 12-mo study, all patients were followed-up for 12 additional months. Normalization or decrease of more than 50% from baseline of serum ALT levels occurred in 42% of treated patients the fourth month of therapy, 26% the twelfth month and 3% the twenty-fourth month vs. 7%, 7% and 0%, respectively, in the control group. However, relapses occurred in 7 of 8 (87.5%) responders 1 to 10 mo (mean = 3.5 mo) after cessation of therapy. Liver biopsies were carried out at baseline and during the twelfth month of treatment. Histological improvement, mostly caused by decrease of portal inflammation, was observed in 57% of treated and 36% of untreated patients. Measures of antiviral activity (serum hepatitis delta virus RNA and intrahepatic hepatitis delta antigen) showed similar levels in treated and control patients. In treated patients the percentage of patients who were negative for HDV RNA never exceeded that of baseline. Although interferon-alpha in the dosage given in this study had no antiviral effect on patients with chronic hepatitis D, it reduced hepatic inflammation as measured by ALT levels. Whether a longer duration or reinstitution of interferon-alpha therapy would achieve long-term control of ALT levels and prevent chronic liver damage is not known.