IT IS axiomatic in the control of disease that the first step is to identify those at high and those at low risk. The characteristics of these two groups often point to methods of preventing, palliating, or even curing the disease. Many biological characteristics-constitutional and environmental-have been implicated as risk factors in the development of ischemic heart disease. These have included various serum lipid constituents, notably cholesterol; dietary habits, both qualitative and quantitative; arterial blood pressure; obesity; use of tobacco; heredity; and physical activity, to mention but a few. It is possible to select evidence to support or to refute nearly every hypothesis suggested. Many of the divergent conclusions stem from differences in study designs and in sampling. It is our admittedly prejudiced feeling that longitudinal studies such as the Cardiovascular Health Center (CVHC) at Albany and the Public Health Service Project at Framingham, Mass., offer the best opportunity to identify and to assess the risk factors in the development of ischemic heart disease because adequately large, well defined population samples are subjected to detailed, serial study. In this paper we examine some of the popularly accepted risk factors in the light of results from our own study. For orientation we wish to quote the late Gordon Myers' description of "The Man Least Likely to Have Coronary Heart Disease"*: