EPIDEMIOLOGY, because of its concern with populations rather than individuals, is able to give unique insights into the etiology and prevention of disease and the influence of man's environment upon his health. The increasingly widespread use in recent years of exogenous estrogenic hormones by relatively healthy women has added a powerful new environmental agent to the influences on the health of large proportions of our population. Much of the needed information on the long-term effects of oral contraceptives and of estrogens used for replacement will be obtained only by epidemiologic studies (1-2). We shall review existing epidemiologic data and present some new data based on U.S. mortality statistics through the year 1967.