Congenital immunodeficiency disorders such as severe combined immunodeficiency disease (SCID), Wiskott-Aldrich syndrome, and Chediak-Hegashi syndrome are almost uniformly fatal with most children dying before age one. Allogeneic bone marrow transplant (BMT) is the treatment of choice. Few of these children have matched donors. We use bone marrow processing techniques that allow us to utilize marrow from the parents. Children who lack HLA-identical donors are offered haploidentical, T-cell depleted parental BMTs. Some of these children do not have an immune deficiency severe enough to allow durable engraftment of processed mismatched bone marrow. Successful engraftment may necessitate the use of immunosuppression. Total body irradiation (TBI) is part of our intensive conditioning regimen for children with Wiskott-Aldrich and Chediak-Hegashi syndrome and most children with SCID who have undergone an unsuccessful prior mismatched, T-cell depleted BMT, or who have a high likelihood of donor marrow rejection based on pre-transplant immune function testing. TBI is considered extremely toxic therapy in infancy, with little information available on the acute and chronic effects. The 10 children presented in this report are among the youngest to have received TBI. Five patients were 2 to 6 months of age when they received TBI. The conditioning regimen for all patients was; antithymocyte globulin (25 mg/kg/day, x 3 days), cyclophosphamide (60 mg/kg/day, x 2 days), and TBI. 7.0 Gy TBI was given as a single dose AP-PA at approximately 15 cGy/min. Half value blocks shielded the brain, eyes and lungs. Six of 10 children were alive from 7 to 72 months post transplant.(ABSTRACT TRUNCATED AT 250 WORDS)