Nearly one third of the world's population have been infected with hepatitis B and the virus is endemic in many Asian countries. With increasing life expectancy and the expected global increase in cancer, chemotherapy induced reactivation of hepatitis B is likely to become an increasing problem. Patients with significant levels of hepatitis B virus (HBV) DNA in serum prior to chemotherapy and patients receiving intensive chemotherapy for hematological malignancies appear particularly at risk. Most patients who suffer reactivation of hepatitis B are positive for hepatitis B surface antigen (HBsAg) prior to chemotherapy and are therefore easily identifiable by routine screening. In addition, the very large population of patients who have been exposed to the virus and have apparently cleared the virus as assessed by serological testing (HBsAg negative/hepatitis B core antibody [HBcAb] positive) may also be at risk of reactivation. These patients should be monitored and in some cases receive prophylaxis during chemotherapy. Published experience with antiviral prophylaxis has largely been limited to the nucleoside analogue, lamivudine. The commencement of antiviral prophylaxis prior to chemotherapy and its continuation until restitution of normal host immunity is the cornerstone to effective prevention of hepatitis B reactivation. This review summarizes the important issues related to HBV reactivation and suggests an algorithm for managing these patients in the clinical setting.