We studied the long-term efficacy of adefovir dipivoxil (ADV) treatment in 42 HBeAg-negative patients with chronic hepatitis B (CHB) who had developed genotypical lamivudine (LAM) resistance with virological and clinical breakthroughs under long-term LAM treatment. Patients were allocated in 2 treatment groups. In the first (n = 14), LAM was switched to ADV monotherapy whereas in the second (n = 28) ADV was added to LAM. The two groups did not differ in patients' characteristics, all of them having HBV genotype D infection with the precore stop codon mutation. Within 12 months from start of ADV treatment, serum HBV DNA became nondetectable and ALT normalized in 71% and 90% of patients, respectively, with no difference between the 2 arms. Patients with baseline HBV DNA levels less than 10(7) copies/ml experienced a significantly earlier and more frequent decline in serum HBV DNA to nondetectable levels as compared with patients with greater than 10(7) HBV DNA copies/ml at baseline (P = 0.0013) This response has hitherto been maintained (median treatment duration 40 months) in all patients with ADV added to LAM, whereas virological and biochemical breakthroughs due to development of ADV signature resistance mutations occurred in 3 of 14 patients (21%) on ADV monotherapy 15 to 18 months from start of treatment (P = 0.0174). Conclusion: Adding ADV to LAM in HBeAg-negative CHB patients with LAM resistance effectively suppresses HBV replication inmost of them and induces biochemical remission that can be maintained in all of them at least for 3 years without any evidence of development of resistance to ADV.