The first partial ileal bypass operation specifically for the reduction of plasma lipids was performed by us in 1963. Since then we have operated upon and followed for more than three months 126 hyperlipidemic patients. Clinical metabolic studies, before and after the procedure, have 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 have been lowered an average 41.1% from the preoperative but postdietary baseline. An average 53% cholesterol reduction has been achieved from a pretreatment baseline using a combination of dietary and surgical management. Plasma triglycerides have been 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, transistory 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. We have not encountered hepatotoxic, lithogenic, or nephrolithiasis complications in our partial ileal bypass patients. Sixty-nine per cent of our patients with preoperative angina pectoris have postoperative improvement or total remission of this symptom complex. Serial appraisal of followup 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 today; it is obligatory in its actions, safe, and associated with minimal side effects.