Endometriosis is a disorder in which tissue (endometrial cells) that normally lines the inside of the uterus grows outside of the uterus. Endometriosis most commonly involves your ovaries, fallopian tubes and the tissue lining your pelvis. Rarely, endometrial tissue may spread beyond the pelvic organs.
With endometriosis, displaced endometrial tissue continues to respond to the hormones of the menstrual cycle--it thickens, breaks down and bleeds with each menstrual cycle. Surrounding tissue can become irritated, eventually developing scar tissue and adhesions — abnormal bands of fibrous tissue that can cause pelvic tissues and organs to stick to each other. When endometriosis involves the ovaries, cysts called endometriomas may form.