We studied the course of virologic breakthroughs detected by a quantitative polymerase chain reaction (PCR) assay in 32 of 78 patients with hepatitis B e antigen (HBeAg)-negative precore mutant hepatitis B virus (HBV) chronic liver disease under long-term lamivudine monotherapy. Serum HBV DNA levels were measured every 3 months and on every biochemical breakthrough. YMDD mutants were detected in 30 of the 32 patients with virologic breakthroughs. Among these 32 patients, biochemical remission rate was 44% at 6 months, 21% at 12 months, and 0% at 24 months after the onset of virologic breakthrough. Development of biochemical breakthroughs was associated with a significant increase of serum HBV DNA levels, which exceeded 100,000 copies/mL in 19 of 20 patients (95%) with biochemical breakthroughs and in only 1 of 8 patients (12.5%) remaining in biochemical remission for at least 6 months after the onset of virologic breakthrough (P < .001). Alanine aminotransferase (ALT) level peaked within 0 to 3 months after the onset of biochemical breakthrough and decreased at 6 months but remained abnormal in all but 2 patients. Follow-up liver histologic lesions in patients with biochemical breakthroughs did not differ from baseline findings, although they were significantly improved in patients remaining in virologic and biochemical remission. In conclusion, the frequent emergence of viral resistance under long-term lamivudine monotherapy in HBeAg-negative precore mutant HBV chronic liver disease is followed by increasing viremia levels culminating in the development of biochemical breakthroughs in most cases. ALT activity peaks close to the onset of biochemical breakthrough, decreasing thereafter but remaining persistently abnormal with fluctuating levels.