Reactivation of viral hepatitis B is a well-known complication in patients receiving chemotherapy or immunosuppressive therapy; its incidence reaches 50%. This reactivation is associated with a high rate of morbidity and mortality. Prophylactic treatment of viral reactivation with lamivudine resulted in lowering the incidence of this risk and improving prognosis in recent studies. Patients with hematological malignancies and other cancers require a precise assessment of their hepatitis B status and adequate management. There is not yet a consensus about the appropriateness of prophylactic treatment of occult hepatitis B infection or of pretreatment virologic assessment, especially measurement of viral DNA.