The fact that estrogens have now been in therapeutic use for more than 25 years made it possible to examine the evidence for their safety and effectiveness in a study of 292 postmenopausal women. The mean duration of estrogen therapy in this group was 5.1 years per patient. The estrogens most used since 1945 have been diethylstilbestrol and conjugated equine estrogens, and the latter have been tolerated without side-effects by almost all patients. Among the commoner symptoms of the menopause, hot flashes were completely relieved in 93 of 94 patients. Postmenopausal osteoporosis was treated by prolonged estrogen therapy in 119 patients, 103 of whom had suffered collapse of vertebrae; either complete or significant relief from pain occurred in 90%. Evidence that estrogen therapy is prophylactic against postmenopausal osteoporosis was obtained in a group of 27 women. A study of the incidence of tumors among the 292 patients yielded no justification for the fear that mammary and cervical carcinoma may result from this therapy.