We report here the first successful transplant of unrelated umbilical cord blood (UCB) for a child with beta-thalassemia major in Taiwan. A total of 2.48 x 10(5)/kg CD34 cells were infused into a 3(1/2)-year-old girl following conditioning with a pre-transplant cytoreductive/immunosuppressive regimen of busulfan and cyclophosphamide. The absolute neutrophil count first exceeded 0.5 x 10(9)/l on day 17, and the patient became red cell- and platelet-independent on days 34 and 49, respectively. Engraftment of donor hematopoietic populations was documented by cytogenetic analysis, hemoglobin electrophoresis, Southern blot analysis for thalassemia markers, and molecular analysis of short tandem repeats sequences. The patient was ex-thalassemic for more than 15 months post transplant. From this experience, it is postulated that UCB will indeed allow us to expand the available donor pool for recipients with an HLA DRB1 mismatched at one locus and reduce the risk of graft-versus-host disease.