Reactivation of resolved hepatitis B virus (HBV) infection has been reported in allogeneic hematopoetic stem cell transplantation (HSCT) recipients, but its epidemiology is not well characterized. We performed a retrospective assessment of the timing and risk factors of HBV reactivation among patients with resolved HBV infection undergoing allogeneic HSCT between January 2000 and March 2008. HBV reactivation was defined as development of positive hepatitis B surface antigen after transplant. Among the 61 patients with resolved HBV infection before transplant (hepatitis B core antibody-positive, hepatitis B surface antigen-negative), 12 (19.7%) developed HBV reactivation. The cumulative probability of HBV reactivation 1, 2, and 4 years after transplant was 9.0%, 21.7%, and 42.9%, respectively. In a time-dependent Cox model, the adjusted hazard ratio (HR) of HBV reactivation for patients with pretransplant hepatitis B surface antibody levels <10 milli-international units per milliliter (mIU/mL) was 4.56 (95% confidence interval [CI] 1.23-16.9) compared to those with levels > or =10 mIU/mL; the adjusted HR among patients who developed extensive chronic graft-versus-host disease (cGVHD) was 7.21 (95% CI 1.25-41.5) compared to those who did not. HBV reactivation is a common late complication among allogeneic HSCT recipients with pretransplant resolved infection. Screening for HBV reactivation should be considered for at-risk HSCT recipients. In this cohort, HBV reactivation often developed in patients with cGVHD. Liver biopsy was useful in those patients with both to delineate the contribution of each to liver dysfunction.