A case of hepatitis B reactivation following bone-marrow transplantation for leukemia in a previously healthy HBsAg carrier is reported. A number of changes in HBV serum markers were contemporary to the acute episode. All of them (increase of HBsAg concentration, conversion from anti-HBe to HBeAg, appearance of anti-HBc IgM and of serum HBV-DNA) were suggestive of a "switching-on" of viral replication. Institution of corticosteroid treatment at the onset of the acute phase did not prevent the fatal outcome.