Killer immunoglobulin-like receptor (KIR) and KIR-ligand (KIRL) interactions play an important role in natural killer cell mediated graft versus leukemia effect (GVL) after hematopoietic stem cell transplant (HCT) for AML. Mathematically accounting for KIR-KIRL interactions may identify donors with optimal NK cell mediated alloreactivity and GVL. In this retrospective study of 2359 donor recipient pairs (DRP) who underwent unrelated donor (URD) HCT for AML, KIR-KIRL interaction scores were determined. Relapse risk was significantly reduced in donor-recipient pairs (DRP) with higher inhibitory KIR-KIRL interaction and missing KIRL (mKIR) scores, with HR=0.86 (P=0.01) & HR=0.84 (P=0.02) respectively. This effect was not observed with activating KIR-KIRL interactions. The inhibitory KIR-KIRL (iKIR) interaction score components were summed to give an inhibitory-missing ligand (IM-KIR) score, which if it was 5 as opposed to <5, was also associated with a lower relapse risk, SHR 0.8 (P=0.004). Acute and chronic graft vs. host disease (GVHD), survival, GRFS and relapse free survival were not significantly different. However, TRM was increased among those with IM-KIR=5, HR, 1.32 (P=0.01). Among those with HLA matched DRP, anti-thymocyte globulin recipients with IM-KIR=5, had a lower relapse rate HR, 0.61 (p=0.001), however TRM was increased in these patients with a HR, 1.49 (p=0.034). This study demonstrates that unrelated DRPs with high inhibitory KIR content scores confer relapse protection, albeit with increased TRM. Clinical trials utilizing donors with a higher iKIR content in conjunction with novel strategies to reduce TRM should be considered for URD HCT recipients with AML to optimize clinical outcomes.