Entecavir switch is one of the treatment options for lamivudine-resistant hepatitis B (HBV) patients in Asia. This study examined the outcome of patients with different baseline resistance genotypes in a cohort study. In this study, 14 patients with chronic HBV were treated with entecavir 1 mg/day for 5 years. Enrolment criteria include: documented lamivudine resistant mutations, treatment with adefovir 10 mg/day for at least 24 weeks, and Child-Pugh score <7. Most had previous failed adefovir therapy and compensated cirrhosis of the liver. Clinical outcomes, liver biochemistries, and HBV DNA were monitored regularly. Patients with virologic breakthrough were rescued with add-on adefovir. At the end of the treatment period, the mean HBV DNA fell from 5.92 × 106 (baseline) to 3.67 × 101 IU/ml. The presence of a HBV polymerase rtM204V mutation at the baseline was found to be the major risk factor for adverse outcomes. Compared to the patients with the rtM204I mutant, patients with the rtM204V mutant had increased risk of virologic breakthrough (80% vs. 0%, P = 0.010) requiring add-on adefovir, slower virologic responses (log rank test, P = 0.0011), failure to reach undetectable HBV DNA levels (60% vs. 0%, P = 0.045), and higher risk of entecavir-resistance (60% vs. 0%, P = 0.045). All the patients with rtM204I and rtA181 mutants had undetectable HBV DNA from 18th month. In summary, lamivudine-resistant HBV patients with the rtM204V mutation have the highest risk of developing entecavir resistance, and entecavir monotherapy should be avoided. Those with the rtM204I and rtA181V mutations may have lower risks, but regular surveillance for viral breakthrough is required. J. Med. Virol. 85:26–33, 2012. © 2012 Wiley Periodicals, Inc.