Abstract One of the following treatments was randomly assigned to 101 consecutive patients with biopsy-proven chronic active hepatitis: prednisone 20 mg daily, azathioprine 100 mg daily, prednisone 20 mg and azathioprine 50 mg daily, or B vitamins 2 tablets daily (control group). Patients were observed at the beginning of the study, then at 2, 6, and 12 months after treatment. At each visit levels of HBsAg and Dane-particle-associated core-antigen (HBcAg) and their corresponding antibodies were determined. 42 patients were HBcAg positive at the beginning of the study. Of these 42 patients, the 34 who were under treatment remained HBcAg positive and showed a rise in HBcAg titre, while the 8 who were not treated had a fall in HBcAg titre (6 patients) or became HBcAg negative (2 patients) in 6 months. Among 59 patients who were HBcAg negative at the beginning of the study, this antigen became persistently detectable in 40% of the 42 patients who were treated, and was transiently present in 2 (12%) out of the 17 untreated patients (p<0·05). Our data indicate that long-term prednisone and/or azathioprine treatments favour the replication of hepatitis-B virus in patients with HBsAg-positive chronic active hepatitis.