Since 1965, there has been a steady decline in the age-adjusted death rates for ischemic heart disease in both sexes, all races and at all ages in the United States. The U.S. decline has been particularly striking during the last five years, yet it has not been similarly observed in most other countries in the world. A variety of explanations have been given for this recent decline including improved methods of diagnosing and treating the coronary patient and changes in lifestyle and diet. Several prospective epidemiologic studies such as the 30 year Framingham Heart Study have developed techniques for identifying those individuals at particularly high risk of developing coronary heart disease using the “standard” risk factors (systolic blood pressure, serum cholesterol level, cigarette smoking and glucose intolerance). Further elucidation of these standard risk factors which explain some, but not all, of the risk for developing CHD, as well as investigation of other endogenous and exogenous variables such as genetic or ethnic differences, geographic differences (altitude and weather), water hardness and fluoridation, industrial exposures and chemical factors such as carbon monoxide and hydrocarbons, and behavioral factors such as coronary-prone (Type A) behavior, will provide a more complete risk profile for CHD. Determination of how these latter variables interact with the standard risk factors and the degree to which they further explain risk for CHD is important to understanding the decline in mortality.