Numerous prospective epidemiologic studies have demonstrated that hypercholesterolemia is a major risk factor for atherosclerotic coronary heart disease. Research on animals indicating that the reduction of serum cholesterol prevented or reversed atherosclerosis had not been shown convincingly in humans. The Coronary Primary Prevention Trial tested the hypothesis that lowering the serum cholesterol in patients who had no existing evidence of coronary heart disease would reduce the subsequent rate of coronary heart disease in those persons.