The presence of antibodies to HBs and HBc antigens indicates previous infection with hepatitis B virus but does not necessarily reflect viral clearance. Immunosuppression such as that observed in patients with bone marrow transplantation may be responsible for viral reactivation followed by acute exacerbation after withdrawal of immunosuppressive therapy. We report a case in a patient with natural immunity to hepatitis B who had undergone allogenic bone marrow transplantation with an identical sibling donor one year before for the chronic myelogenous leukemia in the first chronic phase. Ganciclovir treatment resulted in control of hepatitis virus B replication and in biochemical remission. We suggest that prevention relies on serological evaluation and therapy with active or passive immunisation or antiviral drugs in case of a rapid decline of anti-HBs Ab titers to undetectable levels.