Hematopoietic stem cell transplantation is currently the only curative therapy for beta-thalassemia major. However, <30% of patients have unaffected HLA-identical siblings to serve as donors. We investigated the feasibility of using umbilical cord blood transplants from unrelated HLA mismatched donors and a myeloablative preparative regimen that did not involve total body irradiation. Between October 2003 and November 2004, 5 children with beta-thalassemia major received busulfan, cyclophosphamide, and antithymocyte globulin before cord blood transplantation (median dose, 8.8 x 10(7) cells per kilogram of body weight) from unrelated donors (1 or 2 of 6 HLA antigens were mismatched) and were then evaluated for engraftment, adverse effects, and treatment outcome. The median times to neutrophil engraftment, red blood cell transfusion independence, and platelet engraftment were 12, 34, and 46 days after transplantation, respectively. All patients showed grade II or III acute graft-versus-host disease; none developed extensive chronic graft-versus-host disease until the date of last contact. All patients were alive at a median follow-up of 303 days after transplantation, with complete donor chimerism and transfusion independence. These results are encouraging and clearly show the feasibility of unrelated mismatched umbilical cord blood transplantation in the treatment of children with beta-thalassemia major.