This proposed project takes advantage of comprehensive surveillance data on more than 100,000 women offered breast cancer screening through a program with mailed reminders to schedule mammography examinations within a managed care plan (Group Health Cooperative of Puget Sound, GHC). Cancer outcome (mortality, late-stage disease) for the target population are collected through a Surveillance, Epidemiology, and End Results reporting (SEER) registry and are linked to health care process data (service use, mammography assessments). This proposal includes 3 specific aims: 1) To continue breast cancer data system development at GHC to; a) improve data system software, enhance data storage capabilities, and facilitate data retrieval; b) incorporate new data components pertinent to research, such as a targeted survey; and c) maintain and improve data quality assurance, report generation, and data file development; 2) To use the data system to conduct 5 initiatives: a) The effect of short-term hormone replacement therapy (HRT) cessation on mammographic density; b) The likelihood of additional imaging (mammography and ultrasound) and the associated costs among women stopping HRT compared to women continuing or never using HRT; C) The factors that explain the reduced sensitivity of mammography among younger women; d) The biologic and other factors that influence the likelihood of late-stage disease; and, e) The effect of screening interval on stage at-diagnosis; and 3) To conduct 5 research projects related to screening mammography: a) The additional effect of mammographic breast density on the 5-year risk of breast cancer; b) Screening sensitivity and specificity by phase of menstrual cycle; c) The association between mammographic findings and cancer among women with "probably benign findings"; d) The effect of computer assisted reading on mammography interpretive performance; and e) Biomarkers associated with nodal metastases at-diagnosis among screened women. By continuing our multi disciplinary collaboration and using carefully designed prospective observational and evaluative studies the investigators will contribute to improvements in breast cancer screening, and the understanding of breast cancer biology.