Background/Aims : Entecavir is a synthetic nucleoside analogue, cyclopentyl guanine nucleoside, which has a potent antiviral effect and the least viral breakthrough in hepatitis B virus (HBV) replication. Entecavir has been available in Korea since 2007 but there are few reports on its effects. The aim of this study was to evaluate the virological response (VR) and biochemical response (BR) to entecavir in HBV patients at 3, 6 and 9 months after treatment with entecavir. Materials and Methods : Thirty-three chronic hepatitis B patients who took entecavir for at least 9 months were enrolled. We investigated VR and BR by retrospectively reviewing medical records. Patients who satisfied the following criteria were chosen: 1) initial alanine aminotransferase (ALT) levels = 1.5upper limit of normal (ULN) and 2) initial HBV DNA levels = $5;log_{10};copies/ml$. We measured ALT levels every 3 months until month 9. HBV DNA was measured every 2 or 3 months by polymerase chain reaction (PCR) method. Results : Most patients taking entecavir showed good BR (ALT < 40 IU/L). The BR rates were 61%, 73% and 67% at months 3, 6 and 9, respectively. VR (HBV DNA < $5;log_{10};copies/ml$ or 2 log lower than initial HBV DNA) rates were 82%, 91% and 91% at months 3, 6 and 9, respectively. Undetectable HBV DNA (HBV DNA < 4 log10 copies/ml) rates were 49%, 73% and 85% at months 3, 6 and 9, respectively. Two patients presented with virological breakthrough without adverse effects until month 9. Conclusions : Entecavir showed good BR and VR from month 3 and these effects continued through the 9-month observation period. This suggests that entecavir is also a good choice for the first line treatment of chronic hepatitis B (CHB). Further studies are needed to determine the long-term efficacy and drug resistance of entecavir in Korean CHB patients.