In a Belgian population group of 15 954 male and 2116 female subjects, an epidemiological survey has been conducted to investigate the relationship between dietary fat intake and serum total and HDL-cholesterol. The study has established, both in men and in women, a significant positive relationship between serum total cholesterol and the dietary intake of saturated fat and a significant positive relationship between serum total cholesterol and the dietary intake of saturated fat and a significant negative one between serum total cholesterol and the intake of polyunsaturated fat and the P/S-ration of the diet. At equal levels of consumption, polyunsaturated fat has a decreasing effect on serum total cholesterol which is about half the increasing effect due to saturated fat. Polyunsaturated fat and the P/S-ratio decrease the HDL-cholesterol value but only with borderline significance. The present study demonstrates the importance of dietary fat as a determinant of the serum lipid level within a Population.