Objective: To determine if controlled ovarian hyperstimulation (COH) affects the endometrial expression of IGFBP-1 and IGFBP-3.Design: Prospective, controlled study.Setting: Tertiary infertility clinic.Patient(s): Eighteen oocyte donors undergoing COH cycles and 17 natural cycle controls.Intervention(s): Controlled ovarian hyperstimulation, endometrial biopsies.Main Outcome Measure(s): Immunohistochemical scoring of endometrial IGFBP-1 and -3 expression, morphological endometrial dating, and serum estradiol (E 2 ), LH, and progesterone (P 4 ) concentrations.Result(s): No statistically significant difference was observed between natural and stimulated cycles in change in IGFBP-1 or -3 over standardized cycle days throughout the window of embryo implantation (days 17-24). The IGFBP-1 and -3 expression was zero or near zero for both the natural and COH cycles until day 12-13. Both IGFBPs showed increased production throughout the secretory phase. Advanced endometrial histology (=<1 day) in glands and stroma was noted in COH cycles. Significant positive correlations of E 2 and P 4 were noted with IGFBP-1 and -3 but not with advanced endometrial morphology in the COH cycles.Conclusion(s): The COH cycles have no significantly increased endometrial IGFBP-1 or -3 expression throughout the implantation phase of the luteal cycle compared with normal menstrual cycles. Both IGFBPs were absent in the proliferative phase and increased throughout the secretory portion of the embryo implantation window.