In patients with chronic hepatitis B, brief lamivudine therapy suppresses hepatitis B virus (HBV) DNA but results infrequently in sustained losses of virus replication posttreatment. We evaluated treatment response and its posttreatment durability during up to 18 months of lamivudine therapy (100 mg/d) in 24 patients who had hepatitis B e antigen (HBeAg) despite 1 to 3 months of prior therapy. Therapy was to be stopped after HBeAg loss or seroconversion (acquisition of antibody to HBeAg); posttreatment monitoring continued for 6 months. During therapy, which was well tolerated, HBV DNA became undetectable in all evaluable patients, accompanied by reduced alanine transaminase (ALT) activity. The cumulative 18-month confirmed loss of HBeAg during therapy was 9 of 24 (38%) and seroconversion was 5 of 24 (21%). Therapy was discontinued after HBeAg loss/seroconversion in 7 patients, and HBeAg status was maintained in all. Four of the patients with HBeAg responses lost HBsAg at least once. In 10 (43%) of 23 patients tested, we identified HBV polymerase YMDD mutations, 3 with detectable HBV DNA (2 with ALT elevations) and 7 without virological/biochemical breakthrough. In conclusion, up to 18 months of lamivudine therapy was well tolerated, suppressed HBV replication consistently, and tripled the frequency of HBeAg losses observed during brief-duration therapy; HBeAg loss/seroconversion remained durable posttreatment. The emergence of YMDD-variant HBV was relatively common but occurred typically without reappearance of detectable HBV DNA or ALT elevation. Our observations suggest that lamivudine can be stopped after confirmed HBeAg loss or seroconversion.