Hepatitis B infection is a serious health problem in endemic areas particularly among immunocompromised patients. The more profound immunosuppression in recipients of hematopoietic stem cell transplantations (HCT) can lead to more complicated hepatitis B virus (HBV)-related events. Despite the high risk of recipient infection allogeneic HCT donors with HBV infection are not excluded in the absence of an alternative donor. A 25 year-old man with severe aplastic anemia underwent allogeneic HCT from his HLA-identical sibling. The patient was hepatitis B naive and had normal liver function tests. However the donor had hepatitis B surface antigen (HbsAg) positivity, and collected stem cells were positive for HBV DNA (1 x 10(4) copies/mL). Lamivudine was initiated to treat the patient prior to transplantation. Forty days after the HCT, the patient displayed hepatitis B surface antibody (HBsAb), hepatitis B core antibody (HBcAb), and hepatitis B e antibody (HBeAb), with HBV-DNA negativity. Cyclosporine was tapered and finally stopped at day + 256. On day +368, 112 days after the cessation of cyclosporine HBV reactivation was detected with an HBV-DNA level of 10 x 10(4) copies/mL despite lamivudine. After demonstration of the YMDD mutation, adefovir dipivoxil was combined with lamivudine. The HBV-DNA became negative; AST ALT levels decreased to normal levels after a month of combination therapy. In conclusion adefovir was effective to treat lamivudine-resistant HBV infection in an allogeneic HCT recipient.