Introduction: The concentration of LDL cholesterol (LDLC) is the main parameter used in the stratification of risk and the management of atherosclerosis. Aim of study: The aim of this study was to compare the LDLC concentration calculated by the Friedewald (LDLCF ), the Cordova (LDLCC) and the Planelli (LDLCP ) formulas with the LDLC concentration measured by direct metod (LDLCD) with regard to age and sample matrix. Material and Methods: 594 serum samples were collected from children (1 day to 18 years) hospitalized in University Children’s Hospital in Krakow. Concentrations of hemoglobin A1c and/or creatinine were determined. In all serum samples total cholesterol, triglycerides, high-density lipoprotein cholesterol, apolipoprotein B and LDLCD were measured. In addition, the LDLC concentration was estimated. Results: The means of the LDLCF and LDLCC concentrations were significantly lower compared to the mean of the LDLCD in all samples and in samples from children ≥1 years of age (in all cases p<0,0001). Bland-Altman analys is showed, that the values LDLCC were not comparable to the values of LDLCD in samples from all children and regardless of the LDLCD concentration. In samples with HbA1c≥6,0% mean values of differences between estimated LDLC and LDLCD were higher than in samples with HbA1c<6,0%, regardless of the used formula. The mean values of differences between estimated LDLC and LDLCD in samples with creatinine concentrations <74,0 µmol/l and ≥74,0 µmol/l differed significantly regardless of the used formula (in all cases p<0,001). Conclusion: The differences between concentration of determined and estimated LDLC depend on composition of sample matrix and formula which is used in calculations.