Severe combined immunodeficiency (SCID) is fatal in early childhood if unrecognized and if not treated. The aim was to determine the efficacy of T cell depleted bone marrow transplantation (TCD BMT) in the treatment of children with SCID. Eleven children diagnosed with SCID received histocompatible related donor bone marrow transplantation--HRD BMT (group I). Thirty seven children diagnosed with SCID who did not have histocompatible donors were treated with TCD haploidentical parental bone marrow transplantation (BMT) (group II). TCD was performed by in vitro soybean lectin agglutination followed by E-rosette depletion. Patients were longitudinally assessed for the presence and function of T and B lymphocytes. In group I all children survived. The mean age of children in this group at the time of HRD BMT was 15.4 months. All surviving patients normalized their specific T cell function. Two out of 11 require treatment with intravenous immunoglobulin i.v. Ig. In group II 17 out of 37 (46%) children survived. At the time of TCD BMT the mean age of survivors was 7.5 months, vs. 11.4 months in patients who died. Death was caused most commonly by opportunistic infections, Epstein-Barr virus induced lymphoproliferative disease (EBV-LPD), and graft versus host disease (GvHD). Seventeen out of 17 surviving patients recovered normal numbers of CD3+ cells and antigen specific T cell function. Five out of 17 never recovered their B cell function and require i.v. Ig injections. Early diagnosis, prevention or treatment of opportunistic infections, and enhancement of immune recovery will be necessary to improve survival in patients with SCID treated with TCD BMT.