Administration of immunosuppressive treatment in hepatitis B virus carriers with malignancies is associated with the risk of hepatitis B reactivation. This complication is more frequent in patients with hematologic malignancies because administration of corticosteroids, the mainstay of treatment of these patients, is an independent risk factor for hepatitis B reactivation. When lamivudine is given prior to chemotherapy, it prevents the viral replication during the immunosuppression period; therefore, it might reduce the risk of hepatitis B exacerbation. We performed a prospective study to assess the efficacy of prophylactic administration of lamivudine in this setting. Ten hepatitis B virus carriers with hematologic malignancies were included in this study; seven were HBsAg positive, and three had isolated antiHBc and detectable HBV-DNA levels. Nine patients were given corticosteroids after the administration of lamivudine. Lamivudine was given per os at a dose of 100 mg once daily. In four patients that had not been previously treated with chemotherapy, lamivudine was started 19 days (median) (range, 0–35 days) prior to the onset of chemotherapy. The administration of lamivudine has not stopped since in any of our patients. After a median follow-up of 15 months (range 6–38 months), no hepatitis B reactivation was observed. HBV-DNA levels were decreased in all 6 patients who had detectable HBV-DNA at baseline. Lamivudine was well tolerated. Chemotherapy regimens were administered as planned, and their effectiveness was not compromised by lamivudine. In conclusion, prophylactic administration of lamivudine should be considered as a means of reducing the frequency of hepatitis B reactivation in hepatitis B virus carriers with hematologic malignancies who are being treated with chemotherapy. Am. J. Hematol. 80:197–203, 2005. © 2005 Wiley-Liss, Inc.