alpha v beta 3 integrin expression and pinopod formation have been proposed as a means of distinguishing receptive endometrium from nonreceptive in clinical practice. However, one of the most intriguing facts in infertility is whether one cyclic event may be representative of all patients' cycles, and no study has evaluated the cycle-to-cycle variability of the expression of any of these new proposed markers in human endometrium. We investigated histological dating, alpha v beta 3 integrin expression, and pinopod formation in 45 endometrial biopsy specimens obtained in 15 primary infertility patients. All patients underwent three endometrial biopsies in consecutive spontaneous cycles, regardless of the previous histological findings. All endometrial samples were obtained on postovulatory d 7 as determined by ultrasonography. Agreement between the repeated observations (first vs. second, and first vs. third biopsies) with regard to histological dating and the presence or absence of alpha v beta 3 integrin and pinopods in the endometrium was analyzed using the 2 x 2 frequency tables and Cohen's kappa coefficient. The kappa values ranged from a low of -0.39 (level of agreement, poor) for alpha v beta 3 integrin expression to a high of 0.32 (level of agreement, fair) for biopsy dating by anticipated window of implantation. Overall, these results demonstrate that all endometrial variables investigated had poor reproducibility and high variability cycle to cycle.