Killer immunoglobulin-like receptor (KIR) ligand incompatibility in the graft-versus-host direction was demonstrated to be associated with improved outcome in patients given haploidentical, T-cell-depleted hematopoietic stem cell transplants (HSCTs). The goal of this study was to evaluate whether that observation could be generalized for patients receiving unmanipulated HSCTs from unrelated donors (URD). One hundred thirty patients with hematologic malignancies entered the study. Graft-versus-host disease (GVHD) prophylaxis was uniform and consisted of cyclosporin, short-term methotrexate, and pretransplantation antithymocyte globulin (ATG). Patients were divided into those with (n = 20) and those without (n = 110) KIR ligand incompatibility with respect to their donors. At 4.5 years patients with KIR ligand incompatibility had higher probability of overall survival (87% versus 48%, P =.006) and disease-free survival (87% versus 39%, P =.0007) compared with those without KIR ligand incompatibility. Transplant-related mortality for the 2 groups equaled 6% and 40% (P =.01), respectively. Relapse rates for patients receiving transplants from a donor with or without KIR ligand incompatibility were 6% and 21%, respectively (P =.07). All patients with myeloid malignancies receiving transplants from KIR ligand-disparate donors (n = 13) are alive and disease free. These data indicate that natural killer (NK) cell alloreactivity is associated with better outcome after URD-HSC transplantation when ATG is used as part of GVHD prophylaxis.