The appearance of hepatitis B surface antigen (HBsAg) in patients previously positive for antibody to this antigen (HBsAb) is called reverse seroconversion, a rare complication after hematopoietic stem cell transplantation (HSCT), which occurs almost exclusively after HSCT from an HBsAb-negative donor and the development of chronic graft-versus-host disease (CGVHD). However, we experienced a patient who developed reverse seroconversion 23 months after unrelated HSCT even in the absence of immunosuppressants use or CGVHD. Serum immunoglobulin level was persistently normal. Therefore, all HBsAb-positive recipients should be considered to be at risk for HBV reactivation, even in patients without any risk factors.