This study evaluates the selective progesterone receptor modulator CDB-2914, a man-made hormone. Other progesterone receptor modulators have been shown to block estrogen's stimulating effect on the uterus in monkeys. If CDB-2914 has this effect in menopausal women, it may provide a new approach to hormone replacement therapy. In women studied at the NIH, single doses of CDB-2914 slowed uterine development or induced menses, depending on when it was given during the menstrual cycle. Like raloxifene, CDB-2914 has the potential for use in hormone replacement therapy, but to date the effects of its chronic administration have not been studied. This study aims to evaluate the safety and the physiologic and endocrine effects of chronic oral administration of CDB-2914 in postmenopausal women. All study subjects will receive daily oral estrogen and either CDB-2914, progesterone or a placebo to evaluate whether CDB-2914 blocks or enhances the usual effects of hormone replacement therapy on the uterine lining, lipid levels, clotting factors, bone turnover, hot flashes and quality of life. Weekly blood measurements, quality of life questionnaires, pre- and post-treatment ultrasound examinations of the uterus and a single biopsy of the uterine lining will be performed on all study subjects to assess these endpoints.