To evaluate factors affecting the outcome of sibling and unrelated donor umbilical cord blood transplantation (CBT) in Thai children with beta-thalassemia diseases. The case-series study of all children undergoing such transplants in our institute was conducted Six children with thalassemia major were diagnosed at a median age of 1.5 years and CBT was performed at a median age of 5.5 years (range 2-15). Six donors consisted of three HLA-identical siblings, one two-allele, one three-antigen mismatched sibling, and one one-allele mismatched unrelated cord blood. The median number of nucleated cells infused was 2.83 x 10(7)/kg (range 1.49-5.3); the median number of CD34+ cells infused was 1.94 x 10(5)/kg (range 0.2-5.3). In all, two patients had complete donor engraftment; three had mixed chimerism (MC); one patient died of cerebral thrombosis and neutropenic septicemia. Of the two complete donor-engrafted patients, two developed grade 2 acute graft-versus-host disease (GVHD) which responded well to immunosuppressive therapy. Of the three mixed-chimeric patients, two were clinically cured. With a median follow-up of 7 months (range 2-30), five children survived and have done well with transfusion-independent. Umbilical cord blood provides a reasonable option for hematopoietic stem cell source to transplant for beta-thalassemia diseases and the outcome in the present study was good.