Partial ileal bypass is effective in reducing circulating cholesterol levels. In our study the 10 year effects of the procedure on serum lipids and lipoproteins were studied in 27 patients with heterozygous familial hypercholesterolemia. The ileal bypass patients were compared with conservatively treated case controls matched for age, sex, serum cholesterol level, relative body weight, blood pressure, and smoking habits and also for the existence of diabetes and coronary heart disease. Serum triglycerides were initially slightly higher in the patients undergoing ileal bypass. During the 10 year follow-up eight surgically treated and seven control patients suffered fatal or nonfatal myocardial infarctions. Of these all but one male subject who underwent surgery had had manifest coronary heart disease at entry. Male sex, smoking, triglyceride levels, and angina were significant predictors of new coronary events. The fall in serum cholesterol in patients who underwent ileal bypass and had fatal myocardial infarctions was smaller than in the corresponding subjects without events. The serum lipid levels of the survivors at the end of the 10 year follow-up showed that ileal bypass, as compared with the conservative treatment, had led to a larger decrease in total serum cholesterol (-33% vs -11% in the control patients; p less than .001); lower total serum cholesterol (360 vs 468 mg/dl; p less than .001), low-density lipoprotein (LDL) cholesterol (236 vs 324 mg/dl, p less than .001), and LDL apoprotein B levels (186 vs 231 mg/dl; p less than .001); and higher serum high-density lipoprotein (HDL) (46 vs 38 mg/dl; p less than .05) and HDL2 cholesterol levels (25 vs 16 mg/dl; p less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)