Two male patients, aged 54 years and 17 years respectively, were treated with chemotherapy for non-Hodgkin lymphoma. Both patients were chronic hepatitis-B-virus (HBV) carriers prior to the chemotherapy. One patients died as a result of the virus exacerbating during chemotherapy; the other patient received the antiviral drug lamivudine prior to the chemotherapy and finished the cures without any problems. Exacerbations of HBV replication followed by an increase in serum transaminase activity levels ('flares') occur naturally during the course of the viral infection. However, there is an elevated risk when a patient receives high doses of corticosteroids for a short period, as is the case in chemotherapy for non-Hodgkin's lymphoma. Lamivudine is registered for the treatment of chronic hepatitis B and can be used as a prophylactic prior to chemotherapy or to treat an exacerbation of the hepatitis. It is advisable to systematically test all patients with lymphoma for the presence of the HBsAg. If this is positive, prophylactic administration of lamivudine 100 mg once daily is strongly recommended if chemotherapy is indicated.