During the past two decades prospective epidemiologic studies have established the association between several risk factors and coronary heart disease. Elevations of serum cholesterol and blood pressure, obesity, lack of physical activity and cigarette smoking are among the factors which have been related to an increased risk of heart disease. We are now entering a period in which vigorous efforts will be made to modify these factors in order to prevent the occurrence of heart disease. An important part of this effort will be to determine to what extent these factors are genetically controlled and to what extent they can be modified by manipulation of the individual environment. Recognizing this need, the Field Epidemiological Research Section, National Heart Institute, NIH, has recently embarked on a study of cardiovascular disease risk factors in a large sample of adult male twins. The purpose of the present report is to describe the protocol and methods to be used and the importance of this study to current research in cardiovascular disease.