Case A 25-year-old man with hepatitis B virus (HBV) infection received antiviral treatment with telbivudine 600 mg daily. Six months after starting treatment, the patient developed progressive weakness and myalgia. Physical examination showed symmetrical proximal weakness. Blood tests at admission revealed positive hepatitis b surface antigen (HBs Ag), and, elevated creatine kinase (CK) level (1,614 U/L, normal range: 38-174 U/L). Aspartate aminotransferase was 64.7 U/L (normal range: 8-40 U/L), and LDH was 293 U/L (normal range: 80-285 U/L). Electrodiagnostic studies indicated myopathic changes. A muscle biopsy revealed myositis and no mitochondrial changes were found. Drug-induced myopathy was suspected and telbivudine was changed to entecavir. The muscle weakness and laboratory findings improved. Conclusion A patient developed drug-induced myopathy during long-term treatment with telbivudine for chronic HBV. To promptly detect this reversible adverse event, monitoring of serum CK level and recognition of myopathic signs and symptoms are necessary. Further investigations are needed to clarify the possible mechanism of telbivudine-induced myopathy.