It remains controversial whether alloreactive donor-derived natural killer (NK) cells display graft-versus-leukemia reactions after unmodified allogeneic hematopoietic stem cell transplantation (HSCT). The present study evaluated the role of inhibitory killer immunoglobulin–like receptor (KIR) ligand incompatibility using a well-defined and uniform setting of unmodified allogeneic HSCT in 374 patients with myeloid leukemias. The most striking finding was a significant heterogeneity in the 5-year estimates of hematologic leukemic relapse after human leukocyte antigen (HLA)–identical (n = 237; 22%), HLA class I–disparate (n = 89; 18%), and KIR ligand–incompatible transplantations (n = 48; 5%) (P < .04). Multivariate analysis confirmed that the relative relapse risk (RR) was influenced by HLA class I disparity alone (RR 0.49), but was lowest after HLA class I–disparate, KIR ligand–incompatible transplantations (RR 0.24) (P < .008). The primary graft failure rates, however, increased from 0.4% after HLA class I–identical to 2.3% after HLA class I–disparate, and to 6.3% after KIR ligand–incompatible transplantations, respectively (P < .02). Unlike some other reports, no beneficial effect of KIR ligand incompatibility on other major endpoints of allogeneic HSCT (transplantation-related mortality, and overall and event-free survival) was detectable in the present study. In conclusion, unmodified allogeneic HSCT from KIR ligand–incompatible donors provides a superior long-term antileukemic efficacy in patients with myeloid malignancies.