The impact of the United Network for Organ Sharing mandatory sharing policy on a large transplant center procuring kidneys primarily from Caucasians while serving a pool of prospective recipients composed mainly of blacks is described. This policy requires that all 6-antigen-matched and phenotypically identical donor kidneys be shipped to the appropriately matched recipients. The study consisted of 49 kidneys from 25 cadaveric donors; one kidney was unusable. In general, the 33 recipients of the mandatorily shared kidneys were Caucasian (94%), unsensitized (70%), and first-time transplants (73%). Allograft survival for the 24 first-time recipients was 100% (mean graft survival=11.3 months). Of the 9 regraft kidneys, 2 have failed (mean graft survival = 11.9 months) due to chronic rejection. In comparison, the 16 paired kidneys transplanted into non-6-antigen-matched recipients exhibited a 1-year graft survival of 80% versus 92% for the 33 recipients of mandatorily shared kidneys (P=0.01). These 16 recipients were composed of more blacks (38%), fewer regrafts (6%), and most were unsensitized (75%). All 25 cadaveric donors were Caucasians with very common HLA types. Thus, kidneys provided by the UNOS mandatory sharing policy had excellent allograft survival, and the recipients were largely unsensitized Caucasians receiving their first kidney. The low number of blacks receiving allografts under this policy may be due to two factors. First, the histocompatibility differences between black recipients and the primarily Caucasian cadaveric donor pool limit the number of kidneys available to blacks. Secondly, blacks do not have access to the best-matched kidneys, in part due to few black donors, their best source for well-matched kidneys. Thus, the mandatory sharing program is of clear benefit to the recipients of these well-matched kidneys; however, for a local program servicing a waiting list composed of 64% blacks the policy has been of limited value. In contrast, over 50% of local cadaveric transplants are into black recipients in a waiting time of 197 days, one third the national average for blacks. In conclusion, this study supports efforts to improve graft survival through matching but emphasizes the need to broaden our efforts in all areas of research and organ procurement to serve the entire recipient population, regardless of race. © 1992 by Williams & Wilkins.