Severe combined immunodeficiency (SCID) is a syndrome of diverse genetic cause characterized by profound deficiencies of T- and B-cell function and, in some types, also of NK cells and function. Mutations in thirteen different genes have been found to cause this condition, which is uniformly fatal in the first 2 years of life unless immune reconstitution can be accomplished. In the 42 years since the first bone marrow transplant was given in 1968, the standard treatment for all forms of SCID has been allogeneic bone marrow transplantation. Both HLA-identical unfractionated and T-cell-depleted HLA-haploidentical bone marrow transplants have been very successful in effecting immune reconstitution, especially if performed in the first 3.5 months of life and without pre-transplant chemotherapy. This paper summarizes the longterm outcome, according to molecular type, of 166 consecutive SCID infants given non-conditioned related donor bone marrow transplants at this institution over the past 28.3 years and reviews published reports of longterm outcomes of transplants in SCID performed at other centers.