Hepatitis B virus (HBV) reactivation after allogeneic haematopoietic stem cell transplantation (allo-HSCT) is well known in HBsAg-positive carriers but has only occasionally been reported in patients with resolved HBV infection. We investigated six allo-HSCT recipients with pretransplant anti-HBs and anti-HBc antibodies for serologic markers of HBV infection and for the presence of HBV-DNA in serum. Reverse seroconversion, that is, reappearance of HBsAg after a gradual loss of anti-HBs, but no severe liver damage was observed in three patients at 14, 22 and 12 months after HSCT, respectively. There was an increase in HBV-DNA concentration prior to reverse seroconversion. One patient was repeatedly HBV-DNA positive (10(2)-10(3) copies/ml) without reverse seroconversion. Sequencing of the HBsAg and precore region derived from the four HBV-DNA-positive patients showed no relevant mutations. In conclusion, this study demonstrated a high risk (50%) of reverse seroconversion in allo-HSCT recipients with resolved HBV infection. A highly sensitive HBV-DNA assay (TaqMan-PCR) allowed early identification of the individual patients at risk.