The etiology of endometriosis, a gynecological disease characterized by the presence of endometrial glands and stroma outside the uterine cavity, is still unknown. Our research group has recently demonstrated the presence of ectopic endometrium in human female fetuses at different gestational ages. In this manuscript we describe four new cases of fetal endometriosis found among a series of 52 female fetuses analyzed at autopsy. The anatomical localization of this ectopic endometrium, and its histological and immunohistochemical characteristics are depicted. We suggest that endometriosis is caused by dislocation of primitive endometrial tissue outside the uterine cavity during organogenesis. The clinical and pathological implications of these findings are discussed. J. Cell. Physiol. 227: 1653–1656, 2012. © 2011 Wiley Periodicals, Inc.