Ten patients with evidence of continuing HDV replication were treated with lymphoblastoid alpha-interferon and eight more were followed as a non-randomized control group. Four out of eight patients who completed one year of follow-up had cleared HDV-RNA from the serum whilst none of the control group did so. In these four responding cases there was a transient increase in transaminase levels during treatment and in two, this was followed by improvement. One patient also cleared HBV and seroconverted to anti-HBs (antibody to hepatitis B surface antigen--HBsAg). In one patient with sustained loss of HDV, recurrence of HDV infection was detected 18 months after completion of treatment. These data suggest that alpha-interferon can inhibit HDV replication in the short term but relapse after one to two years may occur. Inhibition of HDV-RNA is associated with improvement in the inflammatory liver disease and now larger studies are required to determine whether it influences survival.