The authors have made a partial ileal bypass on 126 hyperlipidemic patients and report a 3 mth follow up. Clinical metabolic studies, before and after the procedure, demonstrated a 60% decrease in cholesterol absorption, a 3.8 fold increase in total fecal steroid excretion, a 5.7 fold increase in cholesterol synthesis, a 3 fold increase in cholesterol turnover, and a one third decrease in the miscible cholesterol pool. Circulating cholesterol levels were lowered an average 41.1% from the preoperative but postdietary baseline. An average 53% cholesterol reduction was achieved from a pretreatment baseline using a combination of dietary and surgical management. Plasma triglycerides were reduced in primary hypertriglyceridemic patients (type IV) an average of 52.6% from their preoperative but postdietary baseline. One patient died in the hospital and there have been 13 late deaths over the past 10 years. Four cases of postoperative bowel obstruction required reoperation. Diarrhea following partial ileal bypass is, as a rule, transitory and not a significant problem. No appreciable weight loss results from partial ileal bypass, which is an obvious distinction from the results of the far more massive jejuno ileal bypass procedure for obesity. No hepatotoxic, lithogenic, or nephrolithiasis complications were encountered in the partial ileal bypass patients. Sixty nine per cent of the patients with preoperative angina pectoris have postoperative improvement or total remission of this symptom complex. Serial appraisal of follow up coronary arteriographic studies offers preliminary evidence for lesion regression. It is concluded that partial ileal bypass is the most effective means for lipid reduction available; it is obligatory in its actions, safe, and associated with minimal side effects.