We utilized unrelated, partially mismatched (at 1 HLA locus) banked umbilical cord bloods to treat two brothers, aged 1 and 4-1/2 with X-linked lymphoproliferative syndrome (XLP) and one 12 year old boy with X-linked Hyper-IgM syndrome (HIM). XLP was identified by linkage analysis after an older brother died from fulminant liver failure following Epstein Barr Virus(EBV) infection. Both brothers were seronegative for EBV with normal T cell immunity but with hypogammaglobulinemia (IgG of 50 mg/dl at 8 months of age for the younger brother and an IgG of 220 mg/dl at 3-1/2 years of age for the older brother). Pre-transplant immunosuppression and post transplant GVH prophylaxis were used. Neutrophil engraftment occurred 25 and 33 days post transplant and platelet engraftment occurred 53 and 63 days post transplant, respectively. The 1 year old developed prolonged thrombocytopenia 7 months post transplant that eventually resolved with vincristine therapy. HLA studies indicate both B and T cell engraftment. Twenty-one months post transplant, the 1 year old had 2350 CD4 and 1250 CD8 cells/mm3, IgG 717, IgM 171 and IgA 115 mg/dl. Nine months post transplant, the 4 year old had 325 CD4 and 107 CD8 cells/mm3, IgG 995, IgM 75 and IgA 110 mg/dl. Both are off intravenous immune globulin (IVIG) with good antibody responses. The 12 year old with HIM had a CD40 ligand mutation and was clinically well on IVIG (since infancy) but transplanted because of the high risk of hepatic disease that occurs in HIM patients. Four months post transplantation, he has hematopoietic cell engraftment, no evidence of GVH disease, 91 CD4 cells, 46 CD8 cells/mm3, IgG 1270, IgM 24, and IgA 16 mg/dl. He remains on IVIG. There is a normal amount of non-mutated CD40 ligand on the IL-2 stimulated T cells (CD4> CD8) by flow cytometry. The HLA type is that of the donor, indicating successful engraftment. These transplants may prevent the lethal complications of these disorders.