The reverse seroconversion to hepatitis B virus infection has been sporadically described in onco-haematological patients receiving cytotoxic therapy or allogeneic bone marrow transplantation and can be associated with the development of acute icteric hepatitis. We present a male HBsAg-negative, anti-HBc-positive patient with Hairy Cell Leukemia who developed acute B hepatitis more than 1 year after the last course of 2-CdA and 6 months after splenectomy, while the patient was receiving therapy with alphaIFNr. The acute B hepatitis promptly responded to lamivudine therapy followed by viral clearance.