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The magnitude of risk associated with 9/10 mismatched unrelated donor (MMURD) hematopoietic stem cell transplantation and that of mismatches at the individual HLA loci remain unclear. We performed a meta-analysis to assess the difference in clinical outcomes between matched unrelated donor (MUD) and MMURD transplantation. A comprehensive search of Medline and Embase for manuscripts regarding transplantation outcomes in primarily adult patients with hematologic malignancies was performed. The pooled effect estimates were calculated using DerSimonian-Laird random effects models. A total of 13 studies were included, reporting on 13,446 transplants. 9/10 MMURD transplantation was associated with worse overall survival compared to 10/10 MUD transplantation (pooled HR: 1.27, 95% CI: 1.12-1.45; n = 7 studies). Mismatch at HLA-A, -B, or -C was associated with significantly worse overall survival compared to MUD transplantation, while there was no significant difference associated with -DQ or -DPB1 mismatch. Inferior survival associated with HLA-DRB1 mismatch could not be ruled out. Data on acute and chronic graft-versus-host disease were scarce but favored MUD transplantation. In summary, this meta-analysis of the available literature favored MUD over MMURD transplantation in hematologic malignancies and further quantifies the risks associated with specific HLA-allele mismatches. Am. J. Hematol. 91:551-555, 2016. © 2016 Wiley Periodicals, Inc.