Endometriosis is a chronic inflammatory disease associated with altered immune response to endometrial cells facilitating the implantation and proliferation of ectopic endometrial tissues. Although regulatory T (Treg) cells play a key role in T cell-mediated immune response and development of immune disorders, their significance in endometriosis remains to be elucidated. Recently, CD4+CD45RA- forkhead box protein 3 (Foxp3)hi T cells, activated Treg cells, have been identified as a functionally true suppressive population of Treg cells.