Endometriosis is an often painful disorder in which tissue that normally lines the inside of uterus — the endometrium — grows outside uterus (endometrial implant). Endometriosis most commonly involves ovaries, bowel or the tissue lining pelvis. Rarely, endometrial tissue may spread beyond pelvic region. In endometriosis, displaced endometrial tissue continues to act as it normally would — it thickens, breaks down and bleeds with each menstrual cycle. Because this displaced tissue has no way to exit body, it becomes trapped. When endometriosis involves the ovaries, cysts called endometriomas may form. Surrounding tissue can become irritated, eventually developing scar tissue and adhesions — abnormal tissue that binds organs together. Endometriosis can cause pain — sometimes severe — especially during period. Fertility problems also may develop. Fortunately, effective treatments are available.