Reactivation of hepatitis B virus (HBV) is a serious complication of immunosuppressive therapy and cytotoxic chemotherapy. The optimal duration of HBV-DNA monitoring for at-risk patients depends on the clinical features of reactivation, especially the range of potency from therapies to reactivation. We present a case of very late reactivation after chemotherapy for lymphoma and review previous reports of late reactivation cases. We also underscore the significance of developing an indicator for anti-HBV immunity which can be used to determine the optimal monitoring period.