We investigated the serological changes in hepatitis B virus (HBV)-related markers in 55 and 26 hepatitis B surface antigen (HBsAg)-negative patients undergoing allogeneic and autologous stem cell transplantation, respectively, over the past 4 yr. Five of the 17 allogeneic and one of the five autologous patients with pretransplant anti-hepatitis B core antigen antibodies (anti-HBc) were HBsAg-positive after transplantation, whereas none of the patients negative for anti-HBc were HBsAg-positive in both groups. All patients who became HBsAg-positive received steroid-containing immunosuppressive therapy for chronic graft versus host disease (GVHD) or myeloma. Four of the six patients developed flare of HBV hepatitis, and two patients did not. One patient developed fulminant hepatitis treated with lamivudine and plasma exchange. Other five patients received entecavir from the detection of HBsAg. Although HBV-DNA levels became below the limit of detection in all patients, HBsAg positivity remained in three patients after 6 months of treatment. We concluded that anti-HBc positivity is a risk factor for reactivation of HBV after both autologous and allogeneic transplantation, and HBV-related markers should be monitored regularly in these patients. We also stress the efficacy of pre-emptive use of antiviral agents in controlling HBV replication and limiting hepatic injury due to reactivation of HBV in these patients.