The presence and growth of tissue resembling the endometrium outside the uterus. Typical locations include over the tubes, the ovaries, the uterus, the peritoneal lining of the pelvis, the bowel and other unusual areas. The disease is benign but usually progressive. In advanced stages it causes severe scarring of the ovaries and/or tubes which can result in infertility. Even milder forms of the disease whereby only few spots of endometriosis are present are associated with infertility. The cause of endometriosis is not known with certainty, but several mechanisms are involved. More recently, immunological causes have been implicated in the mechanism of infertility in patients with endometriosis. The classic symptoms of endometriosis are cyclic pelvic pain and infertility. Medical or surgical therapy is successful only for the relief of pain. Studies have shown that medical therapy does not improve infertility. Surgery for endometriosis is similarly not successful to treat infertility except in instances where there are mild adhesions partially blocking the tubes.