Hepatitis B virus (HBV) reactivation is a serious clinical problem in HBV carriers undergoing chemotherapy. The clinical course of HBV reactivation can be separated into 2 phases: 1) an increase in HBV replication and 2) hepatic injury. Patients with resolved HBV infections (negative for hepatitis B surface antigen [HBsAg], and positive for both hepatitis B core antibody [anti-HBc] and/or hepatitis B surface antibody) can experience HBV reactivation, and Western guidelines recommend that not only HBsAg but also anti-HBc be screened before initiation of chemotherapy or immunosuppressive therapy. Several meta-analyses have repeatedly confirmed the prophylactic role of lamivudine in preventing HBV reactivation. In conclusion, screening for HBV is required before chemotherapy, and prophylactic antiviral therapy can reduce not only the incidence of HBV reactivation but also HBV-related morbidity and mortality.