Most primary immunodeficiencies (PID) consist in intrinsic defects of lymphocytic and/or phagocytic cell lineages. Therefore, replacement of genetically impaired hematopo'fetic stem cells by normal hematopoietic stem cells is a logical therapeutic approach. First reports of successful bone marrow transplantation (BMT) for PID were published in 1968: Gatti et al.  described correction of a severe combined immunodeficiency (SCID) case and Bach et al.  partial correction of a Wiskott Aldrich syndrome (WAS) case. Since this time, it is estimated that over 1500 patients with PID have undergone allogeneic BMT. About 26 different inherited immune deficiencies have been cured by BMT (Table 1) including the different forms of SCID, of other T cell immunodeficiencies, WAS, various phagocytic cell diseases and more recently hyper IgM and XL-proliferative syndromes. BMT has also been used as a source of mature T cells to correct, at least for some time, the severe T cell lymphocytopenia observed in Di George's syndrome. Based on a better understanding of major histocompatibility complex (HLA) molecular biology and identification of T cell-associated antigens, alternatives to BMT from HLA genetically identical donors have been proposed, i.e., use of related, partially matched donors as well as matched unrelated donors.