After a woman stops menstruating (menopause), her body produces much less of the hormone estrogen. Lower estrogen levels can cause uncomfortable symptoms, such as hot flashes, and put women at risk for thinning of the bones (osteoporosis) and possibly some other medical conditions. Use of the hormone estrogen after menopause (postmenopausal estrogen replacement therapy) can decrease the uncomfortable symptoms of menopause and prevent osteoporosis. Hormone replacement therapy may also decrease the risk for some other conditions, such as heart disease and Alzheimer disease, but these effects are not yet proven. Postmenopausal estrogen replacement therapy has serious but infrequent side effects. Information about these side effects is important to consider in deciding whether hormone replacement is right for a particular woman. Blood clots (also known as thromboembolic disease) are potential side effects of estrogen therapy. Blood clots usually form in the deep veins of the leg, where they can cause pain and swelling. Pieces of blood clots in the leg can break off and travel to the lung, a condition called pulmonary embolism. A person who has pulmonary embolism can become very ill or even die.