Hepatitis B virus (HBV) infection is contagious with transmissiobn vertically or horizontally by blood productsand body secretions. Over 50% of Iranian carriers contracted the infection prenatally, making this the mostlikely route of transmission of HBV in Iran. To evaluate the resistance to adefovir (ADV) therapy in patients withchronic hepatitis B infection, a study was conducted on 70 patients (63 males and 7 females), who had receivedin first line lamivudine and second line adefovir. All were tested for the presence of hepatitis B surface antigen(HBsAg), hepatitis B envelope antigen (HBeAg), serum alanine amino transferase (ALT) level and HBV DNAload before and after treatment with ADV. In all samples, resistance to lamivudine and ADV was tested withreal time PCR. Among seventy patients with chronic hepatitis B infection, 18 (25.7%) were resistant to LAMand 8 (11.4%) were resistant to ADV. Only one patient was negative for the presence of HBS-Ag (5.6%) andtwo were negative for HBe-Ag (11.1%). In this study we used a new method (ALLGIO probe assay) that hashigh sensitivity in detection of adefovir resistance mutants, which we recommend to other researchers. Mutantstrains of the YMDD motif of HBV polymerase can be found in some patients under treatment with lamivudineand ADV. ADV has been demonstrated to be efficient in patients with lamivudine resistant HBV.