The Program on the Surgical Control of the Hyperlipidemias (POSCH), a randomized clinical trial, was designed to test whether cholesterol lowering induced by the partial ileal bypass operation would favorably affect overall mortality or mortality due to coronary heart disease. The study population consisted of 838 patients (417 in the control group and 421 in the surgery group), both men (90.7 percent) and women, with an average age of 51 years, who had survived a first myocardial infarction. The mean follow-up period was 9.7 years. When compared with the control group at five years, the surgery group had a total plasma cholesterol level 23.3 percent lower (4.71±0.91 vs. 6.14±0.89 mmol per liter [mean ±SD]; P<0.0001), a low-density lipoprotein cholesterol level 37.7 percent lower (2.68±0.78 vs. 4.30±0.89 mmol per liter; P<0.0001), and a high-density lipoprotein cholesterol level 4.3 percent higher (1.08±0.26 vs. 1.04±0.25 mmol per liter; P = 0.02). Overall mortality and mortality due to coronary heart disease were reduced, but not significantly so (deaths overall [control vs. surgery], 62 vs. 49, P = 0.164; deaths due to coronary disease, 44 vs. 32, P = 0.113). The overall mortality in the surgery subgroup with an ejection fraction ≥50 percent was 36 percent lower (control vs. surgery, 39 vs. 24; P = 0.021). The value for two end points combined — death due to coronary heart disease and confirmed nonfatal myocardial infarction — was 35 percent lower in the surgery group (125 vs. 82 events; P<0.001). During follow-up, 137 control-group and 52 surgery-group patients underwent coronary-artery bypass grafting (P<0.0001). A comparison of base-line coronary arteriograms with those obtained at 3, 5, 7, and 10 years consistently showed less disease progression in the surgery group (P<0.001). The most common side effect of partial ileal bypass was diarrhea; others included occasional kidney stones, gallstones, and intestinal obstruction. Partial ileal bypass produces sustained improvement in the blood lipid patterns of patients who have had a myocardial infarction and reduces their subsequent morbidity due to coronary heart disease. The role of this procedure in the management of hypercholesterolemia remains to be determined. These results provide strong evidence supporting the beneficial effects of lipid modification in the reduction of atherosclerosis progression. (N Engl J Med 1990; 323:946–55.).