Objective To analyse the effects of controlled ovarian hyperstimulation(COH) on the endometrial expression of collagen fibril(CF) during the peri-implantation period in patients undergoing IVF,and its relation to endometrial receptivity(ER) in repeated implantation failure(RIF).Methods Peripheral blood and endometrial biopsies were obtained from 45 infertile women on days 5,7 or 9 after oocytes retrieval or ovulation in a stimulated cycle(SC) and natural cycle(NC) respectively.CF was assayed by transmission electron microscope and quantified by modified Masson dyeing.The outcome of subsequent embryo transfer(ET) was observed.Results Levels of both E2 and progesterone were higher in the peripheral blood in SC than in NC.Also the expression of CF in the stroma in each secretory phase was increased significantly in SC(P<0.05).After embryo transferring,expression levels of CF in the pregnancy group dropped between the mid-and late-secretory phase,but no change in the non-pregnancy group.In the same term,all patients undergone endometrial curettage had higher pregnancy rate than those without.Conclusion Imbalance of production and degradation of endometrial CF in the secretory phase resulting from COH may be the cause of defective ER and implantation failure in some RIF patients.Endometrial curettage may improve implantation rate by inducing appropriate CF hyperplasia and degradation. Objective To analyze the effects of controlled ovarian hyperstimulation (COH) on the endometrial expression of collagen fibrils (CF) during the peri-implantation period in patients undergoing IVF, and its relation to endometrial receptivity (ER) in repeated implantation failure (RIF). Methods Peripheral blood and endometrial biopsies were obtained from 45 infertile women on days 5, 7 or 9 after oocyte retrieval or ovulation in a stimulated cycle (SC) and natural cycle (NC) respectively. CF was assayed by transmission electron microscope and quantified by modified Masson dyeing. The outcome of subsequent embryo transfer (ET) was observed. Results of both E2 and progesterone were higher in the peripheral blood in SC than in NC. Also the expression of CF in the stroma in each secretory phase was increased significantly in SC (P <0.05). After embryo transferring, expression levels of CF in the pregnancy group dropped between the mid-and late-secretory phase, but no change in the non-pregnancy group. the same term, all patients undergone endometrial curettage had higher pregnancy rate than those without. Conlusion Imbalance of production and degradation of endometrial CF in the secretory phase resulting from COH may be the cause of defective ER and implantation failure in some RIF patients. Endometrial curettage may improve implantation rate by inducing appropriate CF hyperplasia and degradation.