Six patients with chronic type B hepatitis were treated with adenine arabinoside 5′-monophosphate at a dosage of 10 to 15 mg per kg per day for 10 days. All demonstrated an immediate and marked decrease in serum hepatitis B virus DNA and DNA polymerase, and 5 of the 6 became negative for both markers by the end of the period of therapy. One patient remained negative for hepatitis B virus DNA and DNA polymerase when therapy was discontinued. This patient subsequently exhibited clinical, serum biochemical, and histological remission in disease activity concurrent with seroconversion from hepatitis B e antigen to antibody. In the remaining five patients, serum hepatitis B virus DNA and DNA polymerase returned to pretreatment values soon after therapy was stopped, and these patients demonstrated no significant changes in clinical, biochemical, serological, or histological features of the disease. Side effects of the therapy were mild and transient. These results suggest that a 10-day course of adenine arabinoside 5′-monophosphate is not adequate to induce permanent amelioration of infection and disease activity in the majority of patients with chronic type B hepatitis.