Two hormones, estrogen and progestin, are often prescribed to women after menopause to prevent thinning of bones. When these hormones are used in oral contraceptive pills, blood clots are a known side effect of treatment. The blood clots usually develop in the deep veins of the leg, a condition called deep venous thrombosis. Parts of these clots can break off and travel through the bloodstream to the lungs, a condition called pulmonary embolism. Although they are usually treatable with blood thinners, deep venous thrombosis and pulmonary embolism can be disabling or even deadly. Despite these concerns, little is known about the risk for blood clots in women who take both estrogen and progestin as postmenopausal hormone replacement therapy.