Approved hepatitis B virus (HBV) therapies include interferon-alpha and nucleos(t)ide analogues. Lamivudine (LVD) is a nucleoside analogue and following long term LVD therapy, resistance emerges in a significant number of patients. Entecavir (ETV) is a novel deoxyguanosine analogue with potent activity against HBV in chronically infected patients. ETV is highly efficacious in treating nucleoside naive and LVD refractory patients. The aim of the present study was to determine the prevalence of ETV drug resistance in LVD treated/ETV naive (study group) and in untreated naive (control group) patients with chronic B hepatitis. DNA sequencing was applied to 80.-250. amino acid positions on HBV polymerase gene to investigate the ETV resistance and also HBV genotype and HBV polymerase gene overlapped S-gene segment mutations. Primary LVD and ETV drug resistance were detected in 37 (42.6%) and 4 (4.5%) of 87 patients, respectively in the study group. rtT184A, rtT1841 and rtT184S mutations were found related to primary ETV resistance. In these patients also rtL180M and rtQ215S mutations were detected as compensatory mutations and YVDD and YIDD variants were observed at the 204. amino acid codon position. None of the patients in the control group had LVD or ETV resistance. Two of the patients in the study group had mutations at the positions sG145R and sC137G in the overlapped S-gene segment. However, mutations at the overlapped S-gene segment were not affected by the mutations associated with ETV resistance. All of the patients in the study and the control group were of HBV genotype D. The results obtained from this study may guide the treatment choices with ETV in chronic HBV patients treated with LVD and developed resistance to LVD.