The combined effect of clofibrate and a fat-modified diet was determined in 17 hyperlipidemic patients: 7 type IIA, 7 type IV, 2 type VI(IIB), and 1 type III. Control serum lipid levels and lipoprotein patterns and their alteration with a fat-modified diet had been determined previously 1/2 to 2 years for 3 patients, and 6 to 10 years for 14 patients. Two grams of clofibrate a day (0.5 gm four times daily) was taken along with the fat-modified diet for 2 to 6 months by 5 patients and for 2 years by 13 patients. The effect of clofibrate and a fat-controlled diet was also determined in 10 normolipidemic men who were subjects of an 18-day test in which the polyunsaturated fat diet was quantitatively prepared and eaten along with 2 gm clofibrate a day (0.5 gm four times daily). The effect of clofibrate on serum cholesterol levels was a further mean reduction in type IIA patients by 19 %, in type III by 23%, in type IV by 12%, in type VI by 7%, and in normolipidemic subjects by 8%. The extent of the additional serum cholesterol reduction with clofibrate in individual hyperlipidemic patients varied from +10% to minus 44% and was not related directly to the type of hyperlipidemia. The extent of reduction appeared related directly to the level of minus S 40-70 (similar to Sf 12-20) lipoprotein fraction in the control serum sample. Serum triglyceride levels were unaffected in type IIA and normolipidemic subjects. Serum triglyceride levels did not change consistently in the 2 type VI patients, rising by 11% in 1 and dropping by 31% in the other. Serum triglyceride levels were significantly (p = 0.001) and consistently reduced by 39% only in type IV patients.