Reactivation of hepatitis B virus (HBV) has been recognized as one of the most serious complications in patients receiving chemotherapy with rituximab. From October 2007 to December 2008, rituximab was administered to 123 B-cell lymphoma patients in our institute. Four patients with positive hepatitis B surface antigen (HBsAg) received preemptive entecavir, and none of them developed HBV reactivation. For 26 patients whose hepatitis B surface antibody (HBsAb) and/or hepatitis B core antibody (HBcAb) were positive, HBV-DNA was monitored for one year after completion of chemotherapy. During this period, HBV reactivation was observed in two patients. Hepatitis was prevented in one patient by the administration of entecavir at the time HBV-DNA turns positive. Another developed de novo hepatitis B due to failure of monitoring. Preemptive entecavir for HBsAg positive patients and HBV-DNA monitoring for HBsAb and/or HBcAb positive patients seem to be effective.