BACKGROUND: The efficacy of long-term adefovir dipivoxil monotherapy or combination of adefovir and lamivudine in hepatitis B e antigen (HBe-Ag)-negative lamivudine-resistant chronic hepatitis B (CHB) patients is still under investigation. AIM: To assess the safety and efficacy of the long-term adefovir treatment alone or in combination with lamivudine in HBe-Ag-negative CHB patients who had developed breakthrough because of lamivudine-resistant mutants. METHODS: Fifty-nine patients received combination therapy, while 23 switched to adefovir alone after a 3-month course of combination therapy. RESULTS: The median follow-up after adefovir's onset was 31 (18-40) months. Baseline characteristics were similar between the two groups. At 12 and 24 months, 69% and 89% of patients receiving combination therapy and 73% and 82% of patients receiving adefovir monotherapy had serum HBV-DNA <10(4) copies/mL (P > 0.5). Normalization of alanine aminotransferase levels occurred in 81% and 79% of patients receiving combination vs. 61% and 53% receiving adefovir monotherapy at 12 and 24 months, respectively (P > 0.50). Virological breakthroughs because of adefovir-resistant mutants occurred in five patients under adefovir monotherapy and in none receiving combination therapy (P = 0.001). No one developed decompensated liver disease or hepatocellular carcinoma during follow-up. Re-introduction of lamivudine in adefovir-resistant patients achieved reduction in HBV-DNA and biochemical remission, but re-emergence of lamivudine mutants was observed in one patient after 7.5 months. CONCLUSION: In HBe-Ag-negative CHB patients with lamivudine resistance, adding adefovir to continuing lamivudine therapy maximizes anti-viral efficacy because of absence of viral resistance.