Objective To investigate effects of ovarian high response on endometrial mucin-1 (MUC1) and pinopode in peri-implantation phase in controlled ovarian hyperstimulation(COH)cycles.Methods Ovarian high response was deftned as serum E2＞15 000 pmol/L on the day of hCG administration in COH cycle using GnRH agonist and recombinant FSH(n=8).Healthy and fertile women were used as the natural control (n=10).Endometrial biopsies were performed on the day of LH+7/hCG+7.Pinopode formation was observed by scanning electron microscope.Expression of MUC1 was detected with quantitative Real-time PCR and immunohistochemistry.Results In high response group,the lumen surface was covered with variant pinopodes and microvillous.The expression of MUC1 mRNA in high response group was lower than that in the natural control (P＜0.05).Immunostaining for MUC1 protein in glandular and luminal epithelium in high response group was lower than that in the natural control (P＜0.05).Conclusion Asynchronized pinopode appearance and lower expression of MUC1during peri-implantation period were the characteristics of endometrium in high response group,which may provide a clue of decreased endometrial receptivity in the supraphysiological hormone milieu.