Endometriosis, ehn doh mee tree OH sihs, is a disease of the female reproductive system in which clusters of cells from the lining of the uterus invade other areas of the body. The areas most often affected are the ovaries and the walls of the pelvic and abdominal cavities. Endometriosis is most frequently diagnosed in women 20 through 40 years of age who have never been pregnant and who have experienced severe menstrual pain, infertility, or both.