Exacerbation of hepatitis B virus (HBV) is a serious cause of morbidity and mortality in hepatitis B surface antigen (HBsAg)-positive patients undergoing transplantation. Our aim was to evaluate the effectiveness of lamivudine to prevent hepatitis due to exacerbation of HBV in HBsAg-positive patients treated with allogeneic hematopoietic cell transplantation. We studied 20 consecutive HBsAg-positive recipients of allogeneic hematopoietic cell transplantation who received lamivudine 100 mg daily starting one week before transplantation until week 52 after transplantation (group 1). Serial serum alanine aminotransferase and HBV DNA levels were measured before and after transplantation at 4- to 8-week intervals for the first year and then 4- to 12-week intervals. Their virologic and clinical outcomes were compared with 20 case-matched recipients who did not receive any antiviral therapy to HBV (anti-HBV) before and after hematopoietic cell transplantation (group 2). After transplantation, 9 patients (45%) in group 2 and one patient (5%) in group 1 had hepatitis due to exacerbation of HBV (P < .008), with 3 hepatic failures in group 2 and none in group 1. The one-year actuarial probability of survival without hepatitis due to exacerbation of HBV was higher in group 1 than group 2 (94.1% vs. 54.3%, P = .002). By multivariate Cox analysis, preemptive use of lamivudine effectively reduced hepatitis due to exacerbation of HBV (adjusted hazards ratio, 0.09; P = .021). In conclusion, preemptive lamivudine reduced HBV exacerbation. The use of lamivudine with other immunosuppressive regimens to prevent exacerbation of HBV should be further explored.