Background/Aims: Clevudine, a pyrimidine nucleoside analogue, has potent antiviral effects in patients with chronic viral hepatitis B (CHB). We report the efficacy of initial treatment with clevudine in naive patients with CHB living in Daejeon and Chungcheong Province, South Korea. Methods: One hundred five adults with CHB were administered 30 mg of clevudine per day for an average of 51 weeks. We evaluated viral markers and liver biochemistry retrospectively every 3 months. Results: Alanine aminotransferase (ALT), aspartate aminotransferase (AST), and hepatitis B virus (HBV) DNA before the treatment were 184 ± 188 IU/L, 150 ± 138 IU/L, and 7.1 ± 1.2 log copies/mL, respectively. Undetectable rates (< 60 IU/mL) of DNA were 36.2%, 68.9%, 83.6%, 76.2%, and 75.8% at 12, 24, 36, 48, and 60 weeks, respectively. Seroconversion rates were 9.1%, 13.6%, 24.6%, 26.5%, and 26.1% and ALT normalization rates were 64.5%, 78.1%, 87.9%, 90.0% at 12, 24, 36, and 48 weeks, respectively. Six patients (5.7%) had a viral breakthrough. Conclusions: Clevudine is a useful drug in the initial treatment of patients with CHB, with a potent antiviral effect and low incidence of viral breakthrough. (Korean J Intern Med 2010;25:372-376)