BACKGROUND & AIMS: Lamivudine inhibits hepatitis B virus replication. This study investigated 6 months of lamivudine treatment at three doses. METHODS: Fifty-one patients (43% white, 49% Asian) with chronic hepatitis B were randomly assigned to receive 25, 100, or 300 mg of lamivudine orally once daily for 24 weeks with 24 weeks' follow-up. RESULTS: Serum hepatitis B DNA by liquid hybridization decreased in all patients and was undetectable at the end of the treatment in 7 of 12 (58%, 25 mg), 13 of 14 (93%, 100 mg), and 14 of 16 (88%, 300 mg) patients. Of the 36 patients with abnormal alanine aminotransferase (ALT) levels at baseline, 7 of 11 (64%, 25 mg), 5 of 11 (45%, 100 mg), and 5 of 14 (36%, 300 mg) normalized ALT at treatment completion. Quantitative decreases hepatitis Be antigen and hepatitis B surface antigen concentrations were observed at all doses. In most patients, markers of replication returned after treatment. Two patients (4%) were anti-HBe positive at the end of follow-up. Lamivudine was well tolerated. The incidence of adverse events was similar across all dose groups. However, 2 patients developed temporary hepatic decompensation after increase in transaminase levels after treatment. CONCLUSIONS: Lamivudine was well tolerated and induced sustained suppression of hepatitis B replication during treatment in all patients at all doses. These data support investigation of longer treatment durations of 100 mg once daily.