The marked decline in coronary heart disease mortality in the United States since the late 1960s is most likely due in large part to successful efforts at primary prevention, attributable in turn to improved eating habits, better control of blood pressure and a reduction in smoking. Another part of the decline can probably be explained by an improvement in prognosis. It remains to be established if there has been a reduction in morbidity as well as mortality. There are other countries which have experienced declines, though of a lesser degree, while several countries have recorded increases in coronary heart disease mortality. These downward and upward turns, in general, are likewise compatible with the theory that they are caused by corresponding favourable or unfavourable changes in life styles.