Partial ileal bypass (PIB) is a safe, effective, and lasting therapy for the reduction of lipids and lipoproteins in patients with hyperlipidemia. Following PIB, circulating plasma and low-density lipoprotein (LDL) cholesterol fall markedly, while high-density lipoprotein (HDL) cholesterol rises. The average plasma cholesterol lowering is 25% after diet, with a 40% reduction in the LDL-cholesterol fraction; concurrently, the HDL-cholesterol rises about 8%. These effects have been demonstrated to be maintained for up to 20 years. Currently, PIB is being used in the Program on the Surgical Control of the Hyperlipidemias (POSCH), a randomized controlled clinical trial designed to assess the effects of lipid reduction on mortality and morbidity in a postmyocardial infarction population with arteriographically demonstrated coronary atherosclerosis.