To the Editor.
—As Gaston et al1 point out, the hierarchical system favoring HLA matching disadvantages black renal transplant candidates. The impact of mandatory sharing of phenotypically identical kidneys is often underestimated. According to United Network for Organ Sharing (UNOS) policy, all such kidneys must be "paid back" with a kidney from the next suitable donor of the same blood type. Thus, not only is a kidney shared for a white patient (more than 90% of shared "unmismatched" kidneys are used for white patients2), it is being paid back with a kidney also more likely to be used for a white recipient. As the Table demonstrates, the races have markedly different blood-type profiles (kidneys from type O donors are used for type O recipients).The payback policy focuses on distribution of organs among transplant centers, not among patients. Public Law 99-509 (1986) requires UNOS to acquire and allocate