Recent advances in allogeneic hematopoietic cell transplant (allo-HCT) have led to an increasing use of alternative donors, including banked umbilical cord blood (UCB). Despite these advances, acute GVHD (aGVHD) and chronic GVHD (cGVHD) continue to be the leading causes of early and late transplant-related mortality. ABO-mismatch has been frequently reported as a risk factor for GVHD, however, data in the UCB recipients are limited. We hypothesized that as the lymphocytes in the cord blood are thought to be naive, they will therefore be less likely to mediate GVHD. Therefore, we analyzed the impact of ABO-mismatch on aGVHD and cGVHD in recipients of single and double UCB-HCT. In both univariate and multivariate analyses, presence of ABO-mismatch did not have an impact on aGVHD or cGVHD. Whereas ABO-compatible donors are preferred in recipients of URD-HCT, ABO compatibility generally need not be considered in recipients of UCB-HCT