Preemptive lamivudine in lymphoma patients undergoing intensive chemotherapy can effectively prevent chemotherapy-related HBV reactivation. Nevertheless, the safety profile after withdrawal of lamivudine and the impact of rituximab-containing chemotherapy on HBV reactivation has not been defined. To illustrate the necessity of prolonged surveillance after cessation of preemptive lamivudine in lymphoma patients treated with rituximab and chemotherapy, four patients with B-cell NHL carrying HBV received rituximab plus CHOP. Preemptive lamivudine therapy was administered 1 week before chemotherapy until 4 weeks after completion of chemotherapy. Serial serum alanine aminotransferase (ALT), total bilirubin, and HBV-DNA levels were prospectively monitored in three patients. The fourth patient was closely monitored for ALT. The HBV DNA was checked after development of clinical overt hepatitis. The peripheral blood CD20+ B-lymphocyte counts were analyzed periodically in two patients. All of the three patients studied prospectively had virological relapses with surgence of HBV DNA 6-8 months after completion of rituximab-plus-CHOP (R+CHOP) therapy. Two of the three patients had biochemical relapses and one of them developed severe hepatitis. Sequencing for HBV polymerase gene in these patients failed to show evident emergence of lamivudine-resistant mutations. The fourth patient developed a hepatitis flare-up 6 months after completion of chemotherapy. The CD2+ lymphocytes were totally depleted when HBV DNA started to increase. Delayed HBV reactivation can occur in lymphoma patients receiving R+CHOP after withdrawal of preemptive lamivudine. More protracted lamivudine therapy may be an alternative to close monitoring following chemotherapy, and further studies are needed to define optimal duration of lamivudine therapy.