Over the last forty years, epidemiologic studies have made basic contributions to knowledge of the etiology and natural history of the chronic diseases -- cardiovascular diseases, diabetes, and cancers. Findings from the epidemiologic studies have provided practical foundations for the development of prevention and control programs to improve life styles and to control high blood pressure by drugs, when indicated. These developments and their impact on life styles and risk factors have probably contributed to the marked decline in mortality rates from coronary heart disease, stroke, all cardiovascular disease, and all causes throughout the 1970s and 1980s.