Little is known regarding the effect of KIR/HLA incompatibilities (inc.) in the setting of T-replete haploidentical allogeneic hematopoietic stem cell transplantation using posttransplant cyclophosphamide (PTCy). In this retrospective study, the impact of KIR/HLA inc. on clinical outcomes and NK cell reconstitution was studied in a cohort of 51 consecutive patients receiving a T cell–replete haploidentical allogeneic hematopoietic stem cell transplantation after a reduced-intensity conditioning using peripheral blood stem cells as the source of the graft and PTCy as graft-versus-host disease (GvHD) prophylaxis. The NK cell repertoire reconstitution was examined by multiparameter flow cytometry in 34 of these 51 patients from day 0 to day 100 posttransplant. Genetic KIR2DL/HLA inc. were found to be significantly associated with more GvHD (81.2 versus 45.7%, p = 0.01) and less relapse (6.2 versus 42.8%, p = 0.008) in this context. GvHD is associated with increased levels of differentiated and activated NK cells. A significant loss of KIR2DL2/3+ NK cells was observed at day 30 in patients with inhibitory KIR/HLA inc., suggesting that responsive KIR NK cells are particularly targeted by the immunosuppressive PTCy treatment. Further investigations are needed from a larger cohort with an identical clinical approach to consolidate these results and to identify the NK cell subsets that may be beneficial for the graft-versus-leukemia effect observed. Because many haploidentical donors can be identified in a family, the prediction of KIR NK cell alloreactivity could be of crucial importance for donor selection and patient outcome.