Objective To explore the relationship between the endometriosis (EM) associated infertility and changes of CD4+CD25+FoxP3 Tregs in the peripheral blood and eutopic endometrium during peri-implantation phase. Methods Twenty-seven women with EM associated infertility of different stages were selected. The CD4+CD25+FoxP3 Tregs counts in peripheral blood mononulear cells (PBMCs) and the ratio of CD4+T cells and PBMCs were detected by the flow cytometry. The expression of CD4+CD25+FoxP3 mRNA of eutopic endometrium was detected by the RT-PCR. The controls were twenty procreated women without EM (n=20). Results Compared to the controls, the CD4+CD25+FoxP3 Tregs counts in PBMCs and the ratio of CD4+ T cells and PBMCs of women with minor EM were significantly higher. The differences were statistically significant (P<0.05). The CD4+CD25+FoxP3 Tregs counts in PBMCs and the ratio of CD4+ T cells and PBMCs of women with severe EM were significantly higher than those of women with minor EM. The differences were statistically significant (P<0.05). The relative expression of FoxP3 mRNA of eutopic endometrium showed that the expression of women with minor EM was significantly higher than that of controls (P=0.000 1), and the expression of women with severe EM was significantly higher than that of controls and women with minor EM (P<0.000 1). Conclusion The CD4+CD25+FoxP3 Tregs in the peri-implantation phase are not directly relevant to the pathogenesis of abnormal endometrial receptivity of infertile women with EM. CD4+CD25+ FoxP3 Tregs may be relevant to the incidence and development of EM and may aggravate lesions caused by EM and lead to the infertility.