Endometriosis affects up to 10% of women of reproductive age in the U.S. with chronic pelvic pain, dysmenorrhea, and probable increased risk of infertility. Although the disease has been identified as a top research priority by the National Institutes of Health, the etiology is still uncertain, and few modifiable risk factors have been identified. Past epidemiologic research has been inconclusive about the impact of reproductive or contraceptive history on the development of endometriosis. Most studies of these risk factors have been limited by using cases identified at infertility surgery, as these women may be atypical of all endometriosis cases in terms of reproductive and contraceptive history. We propose a population-based case-control study of endometriosis among the entire population of reproductive-age women enrolled in an HMO in western Washington State to test the hypothesis that spontaneous abortion and induced abortion may increase risk of endometriosis, taking into account possible hormonal and immunological influences. The associations between endometriosis and delayed childbearing and various types of contraception will be investigated as well. All 18-44 year old enrollees of Group Health Cooperative of Puget Sound (GHC) diagnosed with endometriosis by surgery, and a portion of those diagnosed by physical examination or ultrasound examination alone between August l, 1996 and April 30, 2001 will be identified and invited to participate (approximately 750 women of all races). Cases will be selected using computerized records, and eligibility verified by medical records. Controls randomly selected from computerized enrollment files will be frequency matched to cases on age and primary care clinic region. Data will be obtained from cases and controls by in-person interview, and anthropometric measurement; and linked with the GHC computerized pharmacy database. Subjects will be interviewed regarding their reproductive and contraceptive histories; including spontaneous and induced abortions, stillbirths, livebirths, use of hormonal contraceptives, intrauterine devices, and sterilization procedures; menstrual history; infertility history; and other risk factors for endometriosis.