The study examines the application of expanded natural killer cells (NK cells) following haploidentical allogeneic hematopoietic stem cell transplantation (haplo-HSCT) for AML or MDS. Haplo-HSCT is a preferred treatment option for patients with AML or MDS without a HLA-matched donor. With administration of cyclophosphamide post-transplant , the safety of the procedure is similar to a HSCT from a HLA-identical donor. Relapse of AML/MDS represents a serious problem following haplo-HSCT. NK cells are immune cells able to destroy tumor cells. Their potency has been established particularly in the setting of a haplo-HSCT. In the current study, study participants undergoing haplo-HSCT will receive expanded NK cells from their respective stem-cell donors following haplo-HSCT. The primary goal of the study is to establish the safety and feasibility of this approach. In addition, the activity of the NK cells will be examined.