14 previously untreated patients with hyperlipoproteinemia type IIa according to Fredrickson underwent a controlled clinical trial. The study was designed to clarify the effects of clofibrate on the bloodlipids. All patients were willing to cooperate. After leaving their usual dietary habits all of them were given for seven to ten days a "prudent" diet. This diet was rich in vitamins; it had a low caloric-content. 20% of the calories consisted of proteins, 35% of carbohydrates (sweets were omitted) and about 45% of fats with a PS-factor of about 2.2. After the initial dietary treatment all patients were given twice daily for 14 days 500 mg clofibrate. During the use of the "prudent" diet the serum-cholesterol-levels decreased in average with 14.8%. Compared to the values at the beginning of the trial the difference was statistically highly significant. The average-values of triglycerides, phosphatides and the relative percentage of HDL, LDL and VLDL remained unchanged. After the patients received clofibrate there was an additional decrease of the serum-cholesterol-values of 14.8%. The total decrease of the cholesterol-levels compared to the initial values amounted to 27.4%. Furthermore, there was a reduction of the phosphatids in the serum of 14.1% compared to the initial value and of 9.9% compared to the value after dietary treatment alone. Under the combined effects of a "prudent" diet with clofibrate the HDL increased significantly. It can be assumed that clofibrate reduces the serum-cholesterol-levels in cases of hyperlipoproteinemia by decreasing the LDL-fraction, which is rich in cholesterol, and by increasing the HDL-fraction.