Changes in endometrial leukocyte subpopulations and most of all in the percentage of uterine natural-killer cells (uNK), during the menstrual cycles, may have a pivotal role in the implantation process. An increase of activated NK cells (CD56dim CD16+ CD69+) in the peripheral blood of patients with a reduced rate of embryo implantation in IVF treatment has been reported elsewhere, but we found, by using flow cytometry, normal endometrial lymphocyte subpopulations in young patients with a history of repeated unexplained implantation failure who were undergoing IVF cycles for idiopathic infertility. © 2007 American Society for Reproductive Medicine.