A patient with HBsAg-positive subacute hepatitis who presented a progressively deteriorating clinical condition was treated with azathioprine (100 mg/day) for eight months. Although the clinical symptoms disappeared and the biochemical abnormalities decreased, the disease nevertheless progressed to chronic active hepatitis. Moreover, azathioprine favoured hepatitis B virus replication since titers of HBsAg, HBeAg and Dane-particle-associated core-antigen, as detected by radioimmunoassay, increased during treatment. In all, this drug was only of moderate use to our patient.