Reactivation of resolved hepatitis B virus (HBV) infection, which is known as reverse seroconversion (RS), has been reported as a rare complication of allogeneic hematopoietic stem cell transplantation. We retrospectively studied HBV serologic markers in 14 recipients with pretransplant anti-hepatitis B surface antigen antibody (anti-HBs). Progressive decreases in anti-HBs titer were observed in all cases. In 12 cases, anti-HBs titer had decreased to under the protective value. RS occurred in seven cases after disappearance of anti-HBs. Although reseroconversion occurred in five cases, two cases remained in an HBV-carrier status after resolution of hepatitis. In the other five cases, RS did not occur even after disappearance of anti-HBs. The actual risks of anti-HBs disappearance and RS were estimated to be 75.0% and 39.8% at 2 years and 100.0% and 70.0% at 5 years, respectively. In conclusion, RS is a late-onset complication with high frequency that can be predicted by careful monitoring of progressive decrease in anti-HBs titer.