Treatment with the hormone estrogen to increase the amount of estrogen in the body. It is given to women who have gone through menopause or to women who have early menopause caused by cancer treatment or by having their ovaries removed by surgery. Estrogen replacement therapy may help relieve symptoms of menopause, such as hot flashes, night sweats, vaginal dryness, and sleep problems. It may also help protect against osteoporosis (thinning of the bones) and lower the risk of breast cancer in postmenopausal women. Also called ERT.