Objective To evaluate the long-term efficacy and safety. of lamivudine therapy for the treatment of chronic hepatitis B and the clinical influence of emergence of tyrosine methionine aspartic acid (YMDD) motif mutation of hepatitis B virus (HBV). Methods This multicenter, double-blind, randomized, placebo controlled trial began in 1996. A total of 429 patients with HBsAg, HBeAg and HBV DNA positives were enrolled. They were randomized to receive either lamivudine 100 mg daily (n = 322) or placebo ( n = 107) on 3 : 1 ratio for the first 12 weeks. Thereafter all patients were offered open label lamivudine treatment and assessed every 4 weeks for a total of 104 weeks. Results After 1 year treatment 72.7% patients (285/392) had a sustained serum HBV DNA response. HBV DNA continued to be substantially suppressed at the second year, except in patients with the emergence of YMDD mutation whose mean HBV DNA levels increased to 86 Meq/ml (bDNA assay) but were much more lower than that of pre-treatment baseline level. Lamivudine therapy resulted in increased HBeAg loss and HBeAg/anti-HBe seroconversion, which were correlated with both baseline alanine transaminase (ALT) levels and also with duration of lamivudine treatment. HBeAg loss was achieved in 26.8% of patients with ALT > 1-fold upper limit of normal at 2 years and in 35.6% and 55.6% of patients with ALT > 2-fold upper limit of normal and ALT > 5-fold upper limit of normal, respectively. For HBeAg seroconversion, these figures were 17.4%, 22.2%, and 33.3% respectively. By the end of 2 years, ALT levels were remained in normal ranges in 50.3% whose ALT were abnormal before treatment, and in 83% whose ALT were normal before treatment. YMDD mutation were developed in 49.7% of the patients. Their serum HBV DNA levels were slightly increased to bDNA median level 86 Meq/ml and 15% of the patients they were ALT exceeded baseline levels. Four patients clinically flared-up and recovered after stop treatment. The adverse drug reactions (ADRs) of lamivudine were mild to moderate, only two patients were reported as drug related severe ADR. Conclusion Sustained HBV replication and clinical improvement could be obtained by the long-term lamivudine therapy with good tolerance and safety.