Reactivation of hepatitis B virus (HBV) infection in subjects receiving cytotoxic treatment for non-Hodgkin's lymphoma (NHL) is well documented. This report describes the case of a 69-year-old male chronic HBV carrier who developed severe flare-up of hepatitis B following chemotherapy for large B-cell NHL. Prior to chemotherapy, the patient had normal liver function tests and was negative for HBV DNA by polymerase chain reaction (PCR) assay. HBV reactivation consisted of a rise in hepatic transaminases (peak alanine aminotransferase=1178 IU/ml), hyperbilirubinemia (7.1 mg/dl), and high levels of serum HBV DNA (63.6 x 10(6) copies/ml). A liver biopsy revealed highly active hepatitis and confluent necroses. Lamivudine treatment (100 mg daily) resulted in rapid loss of hepatitis B virus DNA, resolution of hepatitis, and clinical recovery. The patient is still in remission for NHL. Lamivudine is effective in the control of HBV reactivation following chemotherapy.