Unrelated donor marrow transplantation was undertaken in eight infants with severe combined immunodeficiency (SCID) and two children each with Wiskott-Aldrich syndrome (WAS) and Chediak-Higashi syndrome (CHS) who did not have histocompatible siblings. Donors for three patients were phenotypically matched at all HLA-A, B, Dr, and Dw loci, whereas nine donors were mismatched from the recipients at one of the HLA-A or B loci but phenotypically identical at evaluable D loci. All but one patient received conditioning chemotherapy and/or radiotherapy before infusion of donor marrow, which was not T-cell depleted. Prophylaxis for graft-versus-host disease (GVHD) consisted of methotrexate and prednisone combined with either cyclosporine A (six patients), antithymocyte globulin (five patients), or anti-CD5 ricin A chain immunotoxin (one patient). All patients engrafted with donor cells, and only 4 of 12 experienced any GVHD (1 of 8 SCID, 1 of 2 WAS, 2 of 2 CHS). Two children who developed grade II and two who developed grade III GVHD were successfully treated and all are now alive, off immunosuppressive therapy, with no evidence of chronic GVHD greater than 18 months after transplant. Ten patients are alive with excellent immunoreconstitution ≥1 year to ≥3 years after transplant; actuarial survival is predicted to be 83% with a median follow-up of 2 years. Two children with SCID succumbed to pre-existing opportunistic infection early posttransplant. We conclude that closely matched unrelated donor bone marrow transplantation can correct congenital immunodeficiencies including variants of SCID, WAS, and CHS, with an acceptably low incidence of transplant-related complications, principally GVHD.