POSCH is a prospective, randomized secondary intervention trial examining the effect of maximal lipoprotein modification achieved by partial ileal bypass on overall mortality and the course of coronary heart disease. In the initial 189 surgical patients, total cholesterol levels decreased from 256.7 +/- 2.6 mg/dl to 166.6 +/- 2 mg/dl, and low-density lipoprotein cholesterol levels decreased from 181.5 +/- 2.7 mg/dl to 94.1 +/- 1.7 mg/dl 3 months after operation. These significant decreases were sustained through 5 years of follow-up (p less than 0.001). The total cholesterol level was 29.2 +/- 0.9 percent lower and the low-density lipoprotein cholesterol level was 43.2 +/- 1 percent lower at 5 years compared with the baseline level. Decreases of similar magnitude were seen in each of the common WHO lipoprotein phenotypes. The baseline total cholesterol level was the only significant independent preoperative predictor of the 5 year total cholesterol level (correlation coefficient 0.547; p less than 0.001), and the baseline low-density lipoprotein cholesterol level was the only significant independent preoperative determinant of the 5 year low-density lipoprotein cholesterol level (correlation coefficient 0.599; p less than 0.001). These relationships are expressed by the following equations: 5 year total cholesterol = 0.54 X baseline total cholesterol + 42.3, and 5 year low-density lipoprotein cholesterol = 0.455 X baseline low-density lipoprotein cholesterol + 19.2. The decrease in total and low-density lipoprotein cholesterol levels after partial ileal bypass are greater than reported by any trial of drug or diet intervention, including the Lipid Research Clinics Coronary Primary Prevention Trial which examined cholestyramine. Estimation of the change in total and low-density lipoprotein cholesterol levels after partial ileal bypass can be made based on preoperative lipid analysis and is essential in comparing different therapeutic modalities and assessing the role of partial ileal bypass among strategies aimed at lowering coronary heart disease risk.