Estrogen treatment of postmenopausal women is effective in relieving the symptoms of vasomotor instability and urogenital atrophy; estrogen treatment is effective in preventing accelerated bone loss and osteoporosis in young women following castration, but in postmenopausal women aging is a more important determinant of accelerated bone loss than is decreased estrogen secretion. Low-dose estrogen treatment of postmenopausal women neither prevents nor increases the risk of arteriosclerotic cardiovascular disease or cerebral vascular disease. It cannot be definitively established that estrogen treatment of postmenopausal women causes an increased incidence of breast tumors, but it is clear that such treatment does not prevent these tumors. It is established that estrogen treatment of postmenopausal women increases the risk ratio of endometrial carcinoma.