HBV and HCV infection proceeds to chronic infection, and anti-cancer chemotherapy is contraindicated in patients with decompasated liver cirrhosis caused by both viruses. Reactivation of HBV refers to an increase in the HBV load caused by chemotherapy in a patient with HBV infection. Reactivation of HBV is classified into 2 categories: reactivation in a carrier and reactivation in a patient with resolved HBV infection. Because hepatitis caused by HBV reactivation is more likely to become severe and makes the treatment of original diseases difficult, it is most important to prevent the onset of hepatitis. The basic strategy for prevention and treatment of HBV reactivation associated with powerful chemotherapy regimens should follow the guidelines for the management of hepatitis B virus infection by the Japan Society of Hepatology. Risk of HBV reactivation is determined by HBV infection status and the degree of immunosuppression. HBV infection status is classified into chronic active hepatitis, inactive carrier, and resolved infection. This corresponds, in descending order, to the risk of reactivation. Patients undergoing chemotherapy should be screened for HBV infection. HBsAg levels should be measured in all patients prior to commencement of treatment. In HBsAg-positive patients, HBeAg, anti-HBe antibody, and HBV DNA levels should also be measured. In HBsAg-negative patients, anti-HBc antibody and anti-HBs antibody should be measured. The next step for patients with resolved HBV infection is measurement of HBV DNA levels. In patients with resolved HBV infection whose HBV DNA level is 20 IU/mL or higher, as in inactive carriers, prophylactic therapy should be commenced before chemotherapy. In patients with resolved HBV infection whose HBV DNA level is less than 20 IU/mL, periodic monitoring of HBV DNA should be performed during and after chemotherapy. Preemptive therapy should be started immediately if the HBV DNA level ex- ceeds 20 IU/mL.