The coronary heart disease risk factor profile of Americans has been improving, with decreasing blood pressure and cholesterol levels. However, it is not apparent whether this improvement is due to primary prevention of risk factors through lifestyle choices or to secondary prevention (management) of established risk factors. Since secondary prevention only targets people with high blood pressure or cholesterol levels, the effect should be demonstrated by temporal declines in the upper percentiles (75th, 90th) of the blood pressure and cholesterol distributions. Primary prevention should shift the entire risk factor distribution, including the percentiles in the middle (50th) and lower (10th, 25th) portions of the distribution. Thus, the effectiveness of primary prevention programs can be evaluated by examining temporal changes in the middle and lower percentiles, and the effectiveness of secondary prevention can be evaluated by examining temporal changes in the higher percentiles.