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In countries with active organ transplant programs and wide acceptance criteria for transplant recipients, such as the four Scandinavian countries, the need for organ grafts has outgrown the supply of cadaveric organs. We cannot expect to attain the maximum number of cadaveric donors every year and in every procurement area because the number of donors depends not only on the efficacy of the procurement program but also on the age of the population, incidence of road traffic accidents, treatment facilities, and other factors. The maximum numbers are set as a goal to strive for, however, Countries that participate in an exchange of cadaver organs to obtain well-matched kidneys for uremic patients and life-saving grafts at a suitable time for heart, lung, and liver recipients, must all have the same approximate number of cadaver donors. Major imbalances in the number of donors and also in the number of patients waiting for a transplantation may result in the supply of organ allografts for one country or region being met by the population of another region. With confidence we are looking forward to the impact of intensified public information and education of hospital personnel on the gap between demand and supply of organ allografts.

The histories and present roles of four European Registries are described. Three are transplant sharing organizations, Eurotransplant (ET), Scandiatransplant (ST), and the United Kingdom Transplant Service (UKTS). The Registry of the European Dialysis and Transplant Association is a patient Registry that tracks patients on dialysis as well as after transplantation and follows them from center to center. The transplant organizations collaborate by linking patient records on the EDTA Register. These large central Registries have evolved specialized computer methodologies for recording the follow-up of patients, researching the patient file, and improving the service to centers. The log-rank test is useful for the evaluation of pooled results. Reliability of donor typings can be verified by testing results for the Hardy-Weinberg equilibrium. A method for the prediction of pool size required to find matched recipients is described. Collaborative studies have been carried out in an attempt to discover the factor(s) responsible for the "center effect." New trends in analysis of transplant results will include detailed documentation of rejection episodes. Compliance by centers contributing to the transplant organizations is ensured by the need for organ interchange, whereas the EDTA Registry depends on the directors of centers being motivated by the publication of pooled statistics and results and by the provision of feedback information to the individual units.

The rate of improvement in HLA matching quality with increasing pool size is predicted. The expected frequency of HLA-A,B-compatible matches increases rapidly up to approximately 47% for pool size 1,000 and from then on the increase is moderate, whereas for HLA-A,B-identical matches the increase in frequency is more constant, the expected frequencies being approximately 34 and 52% for pool sizes 1,000 and 2,500, respectively. The expected frequency of HLA-A,B fullhouse-identical matches is 33% at a pool size of 2,500. The waiting time for the first available HLA-identical kidney is estimated. For a random recipient there is a 25% chance of obtaining an HLA-identical match among the first 500 kidney donors. For two cooperating pools, it is calculated at which relative transplantation activity the kidney import to export is balanced and also the decrease in waiting time for an identical match when cooperating is estimated. The results of a kidney exchange simulation are presented and they agree well with the theoretical considerations.