A 35-year-old man with acute promyelocytic leukemia received an allogeneic bone marrow transplant (BMT) in second complete remission. The donor was his 18-year-old brother, a chronic hepatitis B virus (HBV) carrier with detectable serum hepatitis B surface antigen (HBsAg), hepatitis B e antigen (HBeAg) and hepatitis B virus DNA (HBV DNA). Before BMT, the recipient was immune to HBV, with serum antibody to HBsAg (anti-HBs), antibody to HBeAg (anti-HBe) and normal liver function. Examination of the recipient serum drawn 2 months after BMT revealed reverse seroconversion from anti-HBs/anti-HBe to HBsAg/HBeAg, which remained positive thereafter. Upon reducing cyclosporin 6 months after BMT, acute hepatitis occurred with HBV DNA in serum as evidence of active HBV replication; the patient then developed chronic hepatitis which progressed to cirrhosis and hepatic decompensation within 8 months. Transfusion of HBV DNA/HBeAg-positive bone marrow is thus harmful even when the recipient has anti-HBs prior to BMT.