Chronic carriers of Hepatitis B virus (HBV) infection, who are treated for malignant lymphoma, are at high risk of mortality from reactivated HBV infection. We report a case of a 29-year-old male chronic HBV carrier who developed fulminant reactivated HBV infection following intensive chemotherapy for stage IVB large cell B-cell non-Hodgkin's lymphoma associated with extensive central nervous system and bone marrow involvement. Prior to chemotherapy the patient had normal liver function tests and was negative for HBV DNA by semiquantitative PCR assay. Fulminant HBV reactivation was confirmed following clinical deterioration, massive rises in hepatic transaminases (peak alanine aminotransferase = 2,850 U/l), liver biopsy and rising levels of serum HBV DNA. Following treatment with lamivudine 150 mg bd for 18 weeks dramatic and sustained recovery ensued. Symptoms and liver function tests improved within days and HBV DNA became negative within 12 weeks. Our patient later died from relapsed lymphoma but without evidence of reactivated HBV infection. We advise that lamivudine should be considered during intensive chemotherapy treatment of chronic carriers of HBV.