Mortality from coronary heart disease has declined over 40 percent since the late 1960s. Possible explanations for the decline include the observed trends in risk factors such as changes in rates of hypertension, dyslipidemia, and glucose intolerance; lifestyle changes such as improvements in diet and decreases in the levels of cigarette smoking; and better detection and treatment of heart disease. The specific effects of risk factor, lifestyle, or detection and treatment changes on the declining population rates of coronary heart disease morbidity and mortality were unknown in 1990. Secular trend analyses were needed to elucidate the relative contribution of these factors, particularly dietary variables, on population rates of coronary heart disease.