Allogeneic marrow transplantation in patients positive for hepatitis B surface antigen. Twenty patients who were positive for hepatitis B surface antigen (HBsAg) underwent allogeneic marrow transplant for malignancy or other underlying hematologic disease between 1975 and 1986. After transplant, one patient had serologic evidence of hepatitis B virus (HBV) reactivation whereas three patients had evidence of an immune response to HBV. Among four patients with serologic follow-up of 1 year or more, three remained positive for HBsAg and one became HBsAg negative. Six patients (30%) developed clinical evidence of venocclusive disease and seven patients (35%) developed acute graft-versus-host disease involving the liver, but the incidence of these complications was similar to that expected among patients who are not carriers of HBsAg. Three patients died with hepatorenal failure, but all three had venocclusive disease and the contribution of HBV infection to liver failure was unclear. Available liver specimens obtained at autopsy (six patients) or biopsy (two patients) all showed either HBsAg (one specimen) or hepatitis B core antigen (four specimens) or both (three specimens) by immunoperoxidase staining. Although HBV reactivation leading to hepatic failure has been reported among allogeneic marrow transplant recipients as well as other immunocompromised patients, we did not observe an increase in the incidence of severe liver disease after transplant among these 20 patients positive for HBsAg at the time of transplant, and do not consider positivity for HBsAg to be a contraindication to allogeneic marrow transplantation. 