What is endometriosis? Endometriosis is a gynecological disorder effecting millions of women worldwide. This disorder occurs when the endometrium, the inner lining of the uterus, implants abnormally outside of the uterine cavity. The most common sites of endometrial implants are the ovaries, fallopian tubes, the bladder, the intestines, and the ligaments and muscles in the pelvis. The tissue outside of the uterus responds the same way to the menstrual cycle as the lining o