The low density lipoproteins (LDL) is one of the important risk factor of coronary heart disease. Evaluation of LDL by direct method of ultracentrifugation (U-LDL), which is the most proper one, is not available in standard laboratories. Instead the Friedewald's formula is commonly used (F-LDL), which calculates level of LDL applying the values of fasting total cholesterol (CH), cholesterol of high density lipoproteins (HDL) and triglicerydes (TG). This formula can be used only in these samples, in which the level TG is below 4.5 mmol/l (400 mg/dl). The aim of the study was the examination of the accuracy of Friedewald's formula while compared with the concentration of LDL cholesterol determined by ultracentrifugation method and evaluate the possibility of extending the range of triglyceride concentration above 4.5 mmol/l (400 mg/dl) in this formula. The data of the study included 2213 samples of the fasting blood serum of consecutive patients in whom the direct measurement of lipoprotein fraction and calculation by Friedewald's formula were performed in years 1992-1999. All serum samples were analysed by the same standardized procedures according to National Heart, Lung and Blood Institute (NHLBI) guidelines. The data of 29 samples with extremely high value of triglyceride level (TG > 5.6 mmol/l, it is 500 mg/dl) were excluded from analysis. The main results showed that agreement between two methods of LDL evaluation was satisfactory also in extended range of TG 4.5-5.6 mmol/l (400-500 mg/dl). The mean values of U-LDL were significant higher than F-LDL (by mean 0.07 mmol/l it is 2.6 mg/dl) in group with TG < or = 4.5 mmol/l, but non significantly lower (by mean 0.13 mmol/l it is 4.9 mg/dl) in group with TG 4.5-5.6 mmol/l. Linear Pearson's and interclass correlations between U-LDL and F-LDL were high and significant in all analyses.