National allocation of cadaveric kidneys by HLA matching. Projected effect on outcome and costs. BACKGROUND. Although receiving a cadaveric kidney matched at the HLA-A, B, and DR loci enhances graft survival in cyclosporine-treated patients, the value of a national system of kidney allocation based on HLA matching, with the attendant increased likelihood of better matching, is still questioned. Some fear that the costs of a national system are unjustified when only a small fraction of donors would exactly match any of the 16,000 potential recipients anyway. We estimated the effect on graft survival of the use of HLA matching for all allocations of cadaveric kidneys in the United States. METHODS. The graft-survival rates in five mutually exclusive groups of transplants with increasing numbers of HLA mismatches were estimated by partitioning the data for 22,190 first-time recipients of cadaveric kidneys. Overall graft survival was projected as a weighted average with use of the percentages of transplants in the hierarchical groups in recipient waiting pools of various sizes. We compared the benefits and costs of HLA matching in a national system with those of introducing cyclosporine, which was projected to enhance graft survival by 7 percentage points at 10 years. RESULTS. Sharing kidneys nationally on the basis of hierarchical HLA matching was estimated to enhance graft survival by an additional 5 percentage points at 10 years. The anticipated five-year cost of national allocation of kidneys by HLA matching for 7000 recipients, including consideration of the costs of graft removal and dialysis after transplant rejection, would be +4F6.5 million less than the cost of using cyclosporine alone. CONCLUSIONS. The use of an HLA allocation system will not add to the cost of renal transplantation, but it will improve the long-term results to the same extent as cyclosporine. We propose the initiation of a national kidney-sharing system based on hierarchical levels of HLA matches. 