Transplantation using donors with a KIR B haplotype improves relapse-free survival (RFS) irrespective of the recipient KIR haplotype status. Donor (D) and recipient (R) DNA samples from 448 URD transplants were genotyped for 16 KIR loci. KIR gene content was used to identify haplotypes (A or B), from which KIR genotypes were assigned (KIR A/A or KIR B/x). The Kaplan-Meier curves demonstrate RFS for each donor/recipient genotype pairing (A/A into A/A, A/A into B/x, B/x into A/A, and B/x into B/x). RFS was significantly better after transplantation using KIR B/x donors (28% [95% CI: 23-33]; n = 317) compared with A/A donors (17% [11%-24%]; n = 131; P = .003).The recipient KIR genotype had no effect (P = .75).