Background.Expansion of the current program of national sharing of cadaveric kidney allografts is of uncertain benefit, and the logistical barriers to expanding organ sharing are large. This study estimated the improvement in allograft survival from expanding organ sharing in the United States.Methods.A decision analysis based on allograft survival data from cadaveric allograft recipients throughout the United States compared the mean allograft survival resulting from four allograft-sharing strategies: no national sharing, national sharing of allografts matched at 6 histocompatibility alleles, national sharing of allografts matched at 4 or more alleles, and national sharing of allografts matched at 2 or more alleles.Results.Sharing allografts matched at 4 or more alleles was optimal (mean allograft survival=6.35 years). This survival was little better than the mean survival of the other three strategies (no national sharing, 6.21 years; national sharing of allografts matched at 6 alleles, 6.31 years; and sharing of allografts matched at 2 or more alleles, 6.33 years). The increment in the proportion of allografts surviving 4 years or more under the optimal strategy compared with no national sharing was <2%. A similar decision model comparing kidney transplant outcomes before and after the introduction of cyclosporine showed that this drug has had a much greater impact on mean allograft survival than would be expected to occur with national allograft sharing: 6.07 years with cyclosporine versus 3.79 years without cyclosporine.Conclusions.Expanding national allograft sharing would achieve little improvement in mean allograft survival. The limited benefit and logistical barriers to expansion of allograft sharing should be considered before following recommendations to expand the current U.S. allograft-sharing program.