We retrospectively analyzed the development of lymphocytes and of the main immunological functions in 33 patients with severe combined immunodeficiency who survived at least 6 months after bone marrow transplantation (BMT). Eighteen patients received HLA-identical BM and 15 received HLA-nonidentical BM. Development of immune functions occurred faster after HLA-identical BMT as full T- and B-lymphocyte-mediated responses were present at day 186 versus 505, respectively (P = .05). In addition, antibody responses remain completely or partially absent in 8 of 15 patients of the second group. Detection of antibody response after HLA-incompatible BMT correlated with engraftment of donor B cells in informative cases. In patients who received an HLA-nonidentical BMT after chemotherapy (6 of 15), development of immune functions occurred more rapidly and 6 of 6 had B-cell functions, including normal antibody production. Autoimmunity was not uncommon and was found after HLA-incompatible BMT (4 of 15) or after HLA-partially phenotypically identical BMT (2 of 3). Antibodies were in most cases specific for blood cells. Occurrence of autoimmunity correlates with poor B-cell functions and to a lesser extent with defective T-cell responses. This type of study may lead to definition of a more accurate strategy for performing BMT in patients with severe combined immunodeficiency.