Allogeneic hematopoietic transplantation relies on T-cell alloreactions for engraftment and the graft-versus-leukemia (GVL) effect. In human leukocyte antigen (HLA) haplotype-mismatched transplants, extensive T-cell depletion of the graft is essential to prevent GVHD. This raises the question of whether mismatched transplants exert any GVL effect and whether it will ever be possible to reduce the intensity of preparative regimens. Because natural killer (NK) cells are negatively regulated by MHC class I-specific inhibitory receptors, mismatched transplants may trigger NK-cell alloreactivity. HLA class I disparities driving NK-cell alloreactions in the GVH direction mediate strong GVL effects, produce higher engraftment rates, and do not cause GVHD. In murine MHC-mismatched transplant models with no donor T-cell reactivity against the recipient, the pre-transplant infusion of donor-vs-recipient alloreactive NK cells conditioned the recipients to bone marrow transplantation without GVHD. NK-cell alloreactivity may be a unique therapeutic tool for tolerance induction and clearance of leukemia in hematopoietic transplantation.