The aim of the work was to determine the concentration of serum cystatin C and usability of its measurement in diagnostics of renal failure in cats. The reference point was the value of glomerular filtration rate (GFR), calculated on the basis of plasma clearance of iohexol (Pcio). Seventy cats were divided into two groups: group I -24 healthy cats, group II -46 cats affected with renal dysfunction. The measurement of serum cystatin C was done according to immunonefelometric method. In order to estimate Pcio, the following blood samples were collected: pre-sample -0, and samples collected 120, 180, 240, and 300 min after intravenous administration of iohexol. Its concentration was determined using high performance liquid chromatography. Pcio was obtained using a one-compartment model, according to the method developed by Krutzen Back and Nilsson-Ehl. Serum concentration of cystatin C in cats with renal dysfunction was significantly statistically higher (1.3 ± 0.6 mg/L) as compared to the group of healthy animals (0.7 ± 0.2 mg/L). Statistical analysis of the data confirmed strong correlation between GFR and serum concentration of cystatin C (r = -0.51). Data analysis on the basis of the receiver operating characteristic curve proved that cystatin C seems to be more reliable parameter than creatinine in the assessment of kidneys function in cats and the measurement of cystatin C in blood serum can be of a considerable meaning as far as early diagnosis of renal failure in cats is concerned.