Long-term survival rates of human kidney transplants were found to decline at constant rates following the second year after transplantation. The slopes of decline were statistically significantly different for cadaver, parent-to-child, and HLA-identifical sibling transplants, resulting in graft survival half-life times during the constant risk phase of 7.5, 11, and 34 years, respectively. The corresponding patient survival half-life times were 12, 19, and 36 years, respectively. A highly significant difference in long-term survival risk was found when seven transplant centers that had been selected on the basis of their good 1-year graft survival results were compared with seven centers known to have a poor 1-year survival rate. The cadaver graft half-life times during the constant risk phase were 3.3 years for the "poor" centers and 8.7 years for the "good" centers. Thus, although histocompatibility obviously is a main factor in determining long-term survival risk, additional factors such as clinical treatment regimen appear to be influential. Knowledge of the long-term risk constants for the different transplant categories can be applied for the projection of success rates as well as retransplant and dialysis needs.