['To test if nucleolar channel system (NCS) prevalence matches the accuracy of the endometrial receptivity array (ERA) for identification of the window of endometrial receptivity.', 'Comparative retrospective study, May 2008-May 2012.', 'University-affiliated infertility clinic.', 'Forty-nine healthy oocyte donors, regularly cycling, aged 20-34 years with a body mass index of 19-25 kg/m2.', 'Endometrial biopsies were collected throughout the menstrual cycle. All samples underwent transcriptomic signature identification by ERA testing (performed in a prior study) and quantification of NCS prevalence by using indirect immunofluorescence (performed in the present study).', 'Concordance of ERA results determining the window of implantation with NCS prevalence was statistically analyzed using the kappa index. Based on dating according to the luteinizing hormone surge, specimens were dichotomized into receptive (n=24) and nonreceptive (n=25). The NCS prevalence was expressed as percentage of NCSs per endometrial epithelial cells in each endometrial biopsy.', 'Concordance of ERA and NCS dating vs. luteinizing hormone yielded comparable kappa indices of 0.878 and 0.836, respectively. Direct comparison of ERA and NCS dating resulted in a kappa index of 0.796.'] Does timing the window of implantation by nucleolar channel system prevalence match the accuracy of the endometrial receptivity array?