Introduction: The kidneys are a very important pair of organs and have many functions crucial for the life of every human being. Therefore, it is very important to detect renal impairment in time. Today, for this purpose the glomerular filtration rate is determined using endogenous creatinine clearance. However, due to the use of 24-hour urine as a sample to determine engodenous creatinine clearence, whose collection is complicated and often causes wrong results, it is being searched for a new, more sensitive and specific marker of renal function. Cystatin C has great potential and for its determination 24-hour urine collection is not required. Objectives: To estimate whether cystatin C provides useful and sufficient diagnostic information that can be used to assess GFR. Study Design: Cross-sectional study Materials and Methods: The study included 50 patients with impaired renal function and patients with risk of developing kidney disease, 31 of which were woman and 19 men. For the purposes of this study, blood samples were collected in Clinical Hospital Center Osijek. Patients collected 24-hour urine samples at their home. Cystatin C was determined in the blood sample of patients using the immunoassay method on the Beckman Coulter AU680 instrument. The concentrations of creatinine in the serum and in 24-hour urine sample were determined with the enzymatic method on the same instrument. Results: The correlation coefficient (Rho= -0,893) indicates an excellent correlation between cystatin C and GFR. The correlation coefficient (Rho= 0,855) indicates an even better correlation between the endogenous creatinine clearance and GFR. The statistical significance of the difference between the obtained correlations was checked using the Fisher-Z transformation. No statistically significant difference (P= 0,43) was found between cystatin C and endogenous creatinine clearance. Conclusion: Cystatin C can be used to assess GFR instead of the endogenous creatinine clearance, which was used so far.