Hepatitis B virus (HBV) may persist as an occult infection after primary infection despite the emergence of anti-HBs antibody in serum. Immune suppression with gradually reduced titer of anti-HBs can result in enhanced viral replication with development of hepatitis. Here we report 5 patients with positive anti-HBs antibody while negative for HBsAg in serum which developed reverse seroconversion and reactivation of hepatitis after allogeneic hematopoietic stem cell transplantation (HSCT). Patients with lymphoid malignancies are especially at high risk of this complication, probably related to inherently dysregulated immunity in lymphoid malignancies and steroid use during treatment. These patients should be regularly followed for their serum HBsAb after allogeneic HSCT. Disappearance of HBsAb often portends the development of reverse seroconversion and ensuing flare-up of hepatitis. Determination of HBV viral load and preemptive antiviral therapy is indicated for this group of patients to prevent development of full-blown diseases.