HLA disparity between hematopoietic stem cell donors and recipients is one of the most important factors influencing transplant outcomes, but there are no well accepted guidelines to aid in selecting the optimal donor amongst several HLA mismatched donors. In this report, HLA-A is used as a model to illustrate factors that are barriers to delineating the relationship between specific HLA mismatches and transplant outcomes in the United States. Patients in this investigation received transplants for hematological malignancies that were facilitated by the National Marrow Donor Program (NMDP) between 1990 and 2002 (n=4,226). High resolution HLA typing was performed for HLA-A, -B, -C, -DRB1, -DQA1, -DQB1, -DPA1 and -DPB1. HLA-A mismatches were observed in 745 donor-recipient pairs and 62% of these pairs also had disparities at HLA-B, -C and/or -DRB1. The HLA-A mismatches involved 190 different combinations of HLA-A alleles and 51% of these were observed in only one pair. Addition of a single HLA-A disparity when HLA-B, -C, and -DRB1 were matched (n=282) was associated with increased mortality (OR=1.32, CI 1.07-1.63). When HLA-B, -C, and DRB1 were matched, the most frequent HLA-A mismatches were HLAA*0201:0205 (n=28), HLA-A *0301:0302 (n=15), HLA-A *0201:0206 (n=15), HLAA *0201:6801 (n=12), HLA-A*0101:1101 (n=11) and HLA-A*0101:0201 (n=10). There were no statistically significant relationships between any of these disparities and transplant outcomes (engraftment, acute and chronic GVHD, relapse, transplant-related mortality or overall survival) when adjustments for multiple comparisons were considered. Achieving 80% power to detect an effect of any one of these six HLA-A disparities on survival is estimated to require a total transplant population of 11,000 to more than one million U.S. donor-recipient pairs depending upon the HLA disparity. Thus, alternative approaches are required to develop a clinically relevant ranking system for specific HLA disparities in the U.S.