The effect of blood transfusion was analyzed in 194 first cadaver renal transplants and 86 living related renal transplants. The association of blood transfusion with HLA genotyping and poor risk recipients was analyzed. Exclusion of poor risk recipients improved graft survival among the transfused group of patients but not in the small subgroup of nontransfused recipients. No effect of blood transfusion was observed in the living related group. Improved graft survival was observed in both the haplotype-matched and nonhaplotype-matched transfused cadaver recipients. The haplotype-transfused recipients had graft survival rates of 69 and 66% at 1 and 2 years, respectively. The greatest beneficial effect was seen in the double haplotype-transfused cadaver recipients with graft survival rates of 80 and 71% for the same period. The lack of beneficial effect of transfusion in the living related patients is felt to be a result of the fact that the maximum effect had already been achieved by a far superior donor-recipient histo-compatibility than is able to be achieved in a large group of cadaver recipients.