Endometriosis is the abnormal growth of endometrial tissue similar to that which lines the innermost layer of the uterus, but in a location outside of the uterus. Endometrial tissue is shed each month during menstruation. This blood has no way to exit, it becomes trapped and form a cyst called endometrial cyst . Areas of endometrial tissue found in ectopic locations are called endometrial implants. These lesions are most commonly found on the ovaries, the Fallopian tubes, the surface of the uterus, the bowel, and on the membrane lining of the pelvic cavity (i.e. the peritoneum). They are less commonly found to involve the vagina, cervix, and bladder. Rarely, endometriosis can occur outside the pelvis. Endometriosis has been reported in the liver, brain, lung, and old surgical scars. If the endometrial tissue is within an ovary, that ovary will fill with blood. These are called endometriomas and are cysts in the ovary filled with old blood. This old blood has the appearance of thin chocolate.so it is also called chocolate cysts of the ovary. An elevated Ca-125 is often associated with endometriosis. Endometriosis can cause pain – sometimes severe – especially during your period.