The wide use of lamivudine in chronic hepatitis B has produced a monotonic increase in patients with lamivudine resistance. Therefore, treating lamivudine resistance in chronic hepatitis B is a major concern in clinical practice for the treatment of hepatitis B virus (HBV). There is conflicting evidence on the outcome of pegylated interferon alpha (PEG-IFN alpha) therapy against lamivudine-resistant HBV, which is due to mutations in the YMDD motif. We experienced a patient with chronic hepatitis B who was successfully treated with PEG-IFN alpha-2a after the development of virologic and biochemical breakthrough during lamivudine therapy. Virologic breakthrough was associated with the emergence of YMDD mutants 48 months after starting lamivudine therapy. Treatment with PEG-IFN alpha-2a for 12 months resulted in an undetectable serum level of HBV DNA and the resolution of hepatitis, and the virologic response was maintained over 16 months after cessation of PEG-IFN alpha-2a.