<qa>  What is the cause of the disease that is associated with uterine tissue?This study will compare bone mass in women with a history of endometriosis, a disease in which the lining of the uterus grows on nearby tissues, to that of women who have not had endometriosis. Endometriosis may be treated with medication or surgery, or both. Because uterine tissue grows more when estrogen levels are high, medical treatment is designed to lower estrogen. Decreased estrogen, however, is often associated with weak bones and hot flashes. Also, women with endometriosis may have lower bone density as a consequence of their disease. This study will look at bone density in these women, particularly to see if areas other than the lower back may be affected. 