Suppression of the immune system and reconstitution of the donor's immune system may affect the course of a chronic viral infection in the recipients. The aim of this study is to evaluate changes in hepatitis B virus (HBV) serology after bone marrow transplantation (BMT). HBV serology and hepatic function tests were examined in 45 children before and after BMT. Before BMT, 40 patients were HBsAg negative and 5 positive. There were no HBsAg positive donors. HBsAg disappeared in two patients and anti-HBs became positive in one. Donors of these patients were anti-HBs positive. In a third patient, acute HBV infection developed and lasted without complication. This patient also seroconverted to anti-HBs. Anti-HBs disappeared in 7 of 21 anti-HBs positive patients. Among 18 patients who were HBsAg and anti-HBs negative, 11 seroconverted to anti-HBs positivity. Our findings support the notion that having an anti-HBs positive donor is important for adoptive immunity transfer and for preventing HBV replication.