An earlier report identified higher risks of acute and chronic graft-versus-host disease (GVHD) in Caucasian children compared to the Japanese after HLA-matched sibling transplantations. The current analysis explored whether racial differences are associated with GVHD risks after unrelated umbilical cord blood transplantation (UCBT). Included are patients of Japanese descent (n=257) and Caucasians (n=260; 168 of 260 received anti-thymocyte globulin [ATG]). Transplants were performed in the U.S. or Japan between 2000 and 2009; patients were aged 16 years or younger, had acute leukemia, were in complete remission and received myeloablative conditioning regimen. The median ages of the Japanese and Caucasians who received ATG were younger at 5 years compared to 8 years for Caucasians who did not receive ATG. In all groups most transplants were mismatched at 1 or 2 HLA-loci. In multivariate analysis, there were no differences in risks of acute GVHD between the Japanese and Caucasians. However, chronic GVHD was higher in Caucasians who did not receive ATG compared to the Japanese (HR 2.16, p<0.001) and treatment related mortality, higher, in Caucasian who received ATG compared to the Japanese (RR 1.81, p=0.01). Nevertheless, there were no significant differences in overall survival between the three groups.