Lamivudine is a nucleoside analogue with a potent antiviral activity used as prophylaxis against hepatitis B virus reactivation in patients with chronic HBV infection receiving chemotherapy. No standard guidelines exist, however, for the duration of lamivudine treatment. We report a clinical case of a 56-year-old patient with HBeAg-negative cirrhosis who developed a multiple myeloma. He was treated with lamivudine for 1 year while receiving chemotherapy and a subsequent bone marrow transplant. Complete remission from multiple myeloma was achieved. Four months after lamivudine was withdrawn, he experienced HBV reactivation with jaundice, though no YMDD mutations were detected. The patient rapidly developed fatal decompensation with septicemia and renal failure. In conclusion, this case shows that physicians should avoid discontinuing nucleoside therapy in patients with HBV infection who undergo immunosuppression for concomitant neoplastic conditions.