Endometriosis is a painful disorder in which the tissue that lines the inside of the uterus (the endometrium) grows outside the uterus. Endometriosis most commonly involves the ovaries, fallopian tubes and the tissue lining the area of the pelvis. The displaced endometrial tissue continues to act as it normally would — it thickens, breaks down and bleeds with each menstrual cycle. Endometriosis pain can sometimes be severe, especially during menstrual periods. Fertility problems can also develop.