OBJECTIVE: To determine long-term health benefits of nonablative bone marrow transplantation for severe combined immunodeficiency (SCID), we investigated our cohort of 161 related donor bone marrow-transplanted patients with SCID. Only 16 (10%) had HLA-identical donors. STUDY DESIGN: All 124 survivors were sent questionnaires about their current clinical statuses. Details from clinic visits were also compiled. One hundred eleven patients (90%) were reached. We compared outcomes of patients transplanted before and after 3.5 months of life and by molecular defect. RESULTS: The overall survival rate was 77%, but the rate for the 48 infants transplanted in the first 3.5 months of life was 94%, compared with 70% for the 113 transplanted after 3.5 months (P = .002). Twenty-eight (76%) of the 37 deceased patients died of viral infections present at diagnosis. One or more clinical problems were reported to have been present in the past 2 years in 71 (64%) of the survivors, although 95 (86%) were considered healthy by their families. CONCLUSIONS: Most patients with SCID transplanted with related donor marrow without pretransplant chemotherapy have done well in the long term, but those transplanted at <3.5 months of age had a superior survival rate, a lower rate of clinical problems, less need for booster transplants, and better nutritional status.