Although retrograde menstruation is observed in up to 90% of women, endometriosis actually develops in only 15% of women. There is considerable evidence in the literature that ectopic endometrial cells are able to evade immune surveillance and that the immune response in the microenvironment of ectopic lesions is limited. Endometriosis develops when a deficiency in the local immune response has been generated, and progression of the disease is related to the intensity of this process. Over the last couple of decades it has been well known that T regulatory lymphocytes (Tregs) play a crucial role in controlling a variety of physiological and pathological immune responses. In this review we have focused on the physiological alteration of Treg cell infiltration into the endometrium during the reproductive processes of women. We discuss how a disturbance in Treg cell expansion is involved in generating such pathological processes as miscarriage and ectopic pregnancy development. We hypothesize about the role Treg cells might play in the survival of endometriosis foci in ectopic localization and in the evasion of such lesions from host immune surveillance.