A 59-year-old woman received eight cycles of R-CHOP for diffuse large B-cell lymphoma from July, 2004. Pretreatment HBV serological tests were positive, however, the transaminases were within normal limits, and she remained asymptomatic. Lamivudine (LAM) was administered orally from the start of R-CHOP. HBV-DNA levels gradually decreased and were not detected until December, 2004. LAM was administered until 3 months after the end of chemotherapy, with the entire treatment lasting less than 1 year. Two months after the cessation of treatment, she was re-admitted for general malaise. A liver function test revealed severe abnormalities, and HBVDNA levels were increased. Clinically, she was experiencing acute hepatitis due to HBV reactivation. She was then treated with LAM and interferon-beta immediately. Her HBV-DNA levels soon decreased, and liver function improved.