HCT is currently the treatment of choice for children with severe primary immunodeficiencies (PIDs). Frequently, these patients lack an HLA-identical sibling donor, and umbilical cord blood (UCB) transplantation may be an option; however, experience in this field remains scant. Fifteen children with PID (SCID 11, X-linked lymphoproliferative syndrome 2, Omenn's syndrome 1, Wiskott-Aldrich syndrome 1) received a UCB transplant. The donor was unrelated in 14 cases and related in 1. Median age at transplant was 11.6 months (range, 2.9-68.0) and median weight 7 kg (range, 4-21). Thirteen patients were conditioned with busulphan and cyclophosphamide and 2 with fludarabine and melphalan. Nine patients received antithymocyte globulin. Median NC x 10(7)/kg infused was 7.9 (range, 2.9-25.0) and median CD34 x 10(5)/kg 2.9 (range, 1.0-7.9). All patients engrafted. Median days to >0.5 x 10(9)/l neutrophils was 31. Eight patients developed acute graft-versus-host disease (GvHD) grades II-IV and one chronic GvHD. Viral and fungal infections were frequent. Four patients died: three from GvHD grade IV complicated by infection and one from progressive interstitial lung disease. Five-year survival was 0.73+/-0.12. All surviving patients presented complete immunologic reconstitution. No patient is intravenous immunoglobulin (IVIg) replacement therapy-dependent. UCB transplantation is a valid option for children with PID who lack an HLA-identical sibling donor.