Natural killer (NK)–cell alloreactivity in recipients of hematopoietic stem cell grafts from HLA-identical siblings is intriguing and has suggested breaking of NK-cell tolerance during the posttransplantation period. To examine this possibility, we analyzed clinical outcomes in a cohort of 105 patients with myeloid malignancies who received T cell–replete grafts from HLA-matched sibling donors. Presence of inhibitory killer cell immunoglobulin-like receptors (KIRs) for nonself HLA class I ligands had no effect on disease-free survival, incidence of relapse, or graft-versus-host disease. A longitudinal analysis of the NK-cell repertoire and function revealed a global hyporesponsiveness of NK cells early after transplantation. Functional responses recovered at approximately 6 months after transplantation. Importantly, NKG2A⁻ NK cells expressing KIRs for nonself HLA class I ligands remained tolerant at all time points. Furthermore, a direct comparison of NK-cell reconstitution in T cell–replete and T cell–depleted HLA-matched sibling stem cell transplantation (SCT) revealed that NKG2A⁺ NK cells dominated the functional repertoire early after transplantation, with intact tolerance of NKG2A⁻ NK cells expressing KIRs for nonself ligands in both settings. Our results provide evidence against the emergence of alloreactive NK cells in HLA-identical allogeneic SCT.