Renal recipients with hepatitis B virus (HBV) should be treated with prolonged lamivudine. However, lamivudine resistance usually results after YMDD mutation within the HBV polymerase gene. Thereupon, the aim of this study was to investigate the genotypic resistance to lamivudine among renal transplant recipients to identify its effect on the clinical course of HBV in these patients. Between March 1997 and September 2003, eight of 17 renal transplant patients with hepatitis B virus were enrolled into this study and treated with 100 mg of lamivudine once each day. We amplified a selected region of the polymerase gene of HBV in order to confirm mutations in the YMDD motif. Mutations of YMDD region were observed in five of eight patients (62.5%). Out of five patients positive for HBV DNA, three (60%) showed genotypic resistance (YMDD mutation) with a normal ALT level. Two patients converted to HBV DNA negative. But, they were not associated with HBeAg seroconversion. Out of three patients who were pretransplant HBV DNA negative, genotypic resistance was observed in two patients (67%) revealing both positivity of HBe antibody and negativity of HBV DNA. In conclusion, although a normal ALT level and HBV DNA negative are maintained, the mutation of the YMDD locus may develop. Accordingly, we suggest that if the YMDD mutation is not involved in the progression of hepatitis B, lamivudine therapy should be continued despite genotypic resistance.