We report a case of autoimmune thrombocytopenia associated with acute reverse seroconversion of hepatitis B in a patient who was initially hepatitis B virus surface antigen negative and hepatitis B virus surface antibody positive. Reactivation occurred 9 months after chemotherapy with anti-CD 20 monoclonal antibodies and autologous hematopoietic stem cell transplantation for lymphoma had been performed. After non specific polyglobulin injections and treatment with adefovir dipivoxil, thrombocytopenia and viral replication were controlled. Seroconversion for both HBe and HBs occurred at 5 months. Adefovir was stopped 4 months later with no relapse during fifteen months of follow-up. This case shows that patients who have had previous contact with hepatitis B virus should be monitored if they become immunosuppressed, even if anti-HBs were initially present.