BACKGROUND/AIMS: Entecavir (ETV) is effective and safe antiviral agent against hepatitis B virus (HBV) in clinical and real-world setting but, most studies were performed in single institute or have limitation in patient's number. A large-scale nation-wide real-world surveillance study was carried out to investigate safety, efficacy and clinical effectiveness of ETV in Korean patients with chronic hepatitis B (CHB). METHODS: Between 2006 and 2012, 3,444 patients were enrolled from 132 Korean institutions. For the safety assessment, investigators recorded the occurrence of observed and patient-reported adverse events (AEs), as well as laboratory abnormalities. Efficacy, which consisted of change in HBV DNA and alanine aminotransferase (ALT), was evaluated in patients who had received at least 16 weeks of ETV treatment. Overall clinical effectiveness, based on improvement of ALT, HBV DNA and patient's symptoms, was evaluated by physicians. RESULTS: Of the patients, 3,367 were evaluated for safety and 3,115 for efficacy and clinical effectiveness. Three hundred and eighty AEs were reported in 255 cases (7.57%), and 67 adverse drug reactions in 54 cases (1.6%). Serious AEs (SAE) were 19 events in nine cases (0.27%). Serious adverse drug reactions (SADR) were three events in two cases (0.06%), and unexpected SAE/SADR were three events in two cases (0.06%). Medical history and concomitant medications were factors inf luencing incidence rates of AEs. Overall clinical effectiveness rate was 96.53%, which was clinically assessed as marked improved or improved state. CONCLUSIONS This study showed that ETV was well tolerated and clinically effective in Korean patients with CHB in a real-world nation-wide setting.