Responses to conventional donor lymphocyte infusion for post-allogeneic hematopoietic cell transplantation relapse are typically poor. Natural killer cell-based therapy is a promising modality to treat post-HCT relapse. This ongoing phase I trial of adoptively transferred cytokine induced memory-like (CIML) NK cells in patients with myeloid malignancies relapsed after haploidentical HCT saw a dynamic evolution of the activated CIML NK cell phenotype, superimposed on the natural variation in donor NK cell repertoires.