ObjectiveTo identify the differential expressed proteins,and to investigate the relationship between altered expression of annexin A4 during window of implantation [ WOI ( at day-6 after ovulatory day )] in infertile patients with endometriosis and endometrial receptivity.MethodsTwo-dimensional fluorescence differential in-gel electrophoresis (2D-DIGE) and matrix-assist laser desorption ionization-time of flight-mass spectrometry (MALDI-TOF-MS) were used to detect protein expression in endometrial WOI in 10 infertile cases with endometriosis as endometriosis group and 10 infertile cases with tubal factors as control group.The semi-quantitative validation of annexin A4 in the eutopic endometrial tissue during WOI was analyzed by western blot.Results By comparing protein profiles,there were 7 meaningful differential proteins during WOI in infertile patients with endometriosis.One protein with an isoelectric point of 5.84 and relative molecular weight of 36 100 were down regulated 348％ in samples of endometriosis group.It was identified as annexin A4 by mass spectrometry.By western blot,relative intensity of annexin A4 in endometriosis group was 7.2 ±0.9,which was lower than 17.8 ± 2.6 in control group significantly (t =7.654,P =0.002 ).ConclusionLower expresssion of annexin A4 during WOI in infertile patients with endometriosis might be associated with the decrease of endometrial receptivity.