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We have recently described a specialized subset of human natural killer (NK) cells with a CD56dimCD57+NKG2C+phenotype that expand specifically in response to cytomegalovirus (CMV) reactivation in hematopoietic cell transplant (HCT) recipients and exhibit properties characteristic of adaptive immunity. We hypothesize that these cells mediate relapse protection and improve post-HCT outcomes. In 674 allogeneic HCT recipients, we found that those who reactivated CMV had lower leukemia relapse (26% (17–35%), P=0.05) and superior disease-free survival (DFS) (55% (45–65%) P=0.04) 1 year after reduced intensity conditioning (RIC) compared with CMV seronegative recipients who experienced higher relapse rates (35% (27–43%)) and lower DFS (46% (38–54%)). This protective effect was independent of age and graft-vs-host disease and was not observed in recipients who received myeloablative regimens. Analysis of the reconstituting NK cells demonstrated that CMV reactivation is associated with both higher frequencies and greater absolute numbers of CD56dimCD57+NKG2C+NK cells, particularly after RIC HCT. Furthermore, expansion of these cells at 6 months posttransplant independently trended toward a lower 2-year relapse risk. Together, our data suggest that the protective effect of CMV reactivation on posttransplant relapse is in part driven by adaptive NK cell responses.