Despite this encouraging and enigmatic decline in mortality attributed to coronary heart disease in the United States and certain other countries, a paucity of population-based data exist to ascertain whether this temporal decline in death rates is due to changes in the incidence rates of new coronary events, changes in survival after an acute coronary episode or combinations thereof. These data are of crucial importance for formulating explanations for these declining mortality rates. A decline in the incidence rates of coronary heart disease would suggest that the mortality decline is due to measures of primary prevention and changes in the prevalence and/or levels of the major coronary risk factors such as, increased detection and control of hypertension, decreased cigarette smoking and declining serum cholesterol levels. If, however, the incidence rates of coronary heart disease are remaining steady or even increasing, a likely explanation of this downward trend in cardiovascular mortality would be that it is due to effort directed at secondary prevention including the establishment and more effective utilization of coronary care units, therapeutic interventions, and emergency medical services.