Endometriosis is an often painful disorder in which tissue that normally lines the inside of your uterus the endometrium grows outside your uterus (endometrial implant). Endometriosis most commonly involves your ovaries, bowel or the tissue lining your pelvis. Rarely, endometrial tissue may spread beyond your pelvic region. Endometriosis can cause pain sometimes severe especially during your period. Fertility problems also may develop.