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Many cytotoxic agents used in antitumor therapies can cause a kidney damage. Therefore there is a necessity for regular kidney function monitoring before and during chemotherapy. Clearance tests have been proven to be the most accurate indicators for clinical assessment of glomerular filtration (GFR). Because of inconvenience of exogene markers (inulin, ⁵¹Cr-EDTA), endogenous creatinine clearance (ECC) is used in everyday routine work. Results obtained from ECC analysis can often be misleading due to inaccurate urine collec­ting. In order to avoid the inconvenient urine collecting, we are trying to find an easier and faster way of kidney function assessment. Cystatin C is a serum protein, which is a member of the cysteine proteinase inhibitors. It is produced by all nucleal cells and its production rate is independent of sex, nutrition sta­tus, acute and chronic diseases and cancer. Because it is freely filtered at the glomerulus, its serum concentration correlates with glomerular filtration rate. The purpose of this research was to find out if the serum cystatine C concentration cor­relates with GFR and ECC respectively. We analysed 117 serum samples taken from 56 patients. Serum and urine creatinine con­centrations were determined by method based on Jaffe reaction, ECC was calculated from urine flow,  surface area and the ratio between serum and urine creatinine concentrations. We used particle enhanced turbidimetric immunoassay (PET) for cystatin C determination. Data were statistically analysed with SimStat computer program. We used Pearson cor­relation method with 95% confidence interval. A good correlation coefficient (r = 0.736) bet­ween the values of serum creatinine and cystatine C was established within the whole concentration interval. The relation between cystatin C and ECC values was determined with correlation coefficient r = 0.792. Cystatin C differed from ECC values in 8 of 117 samples (6.8%). Methods don’t show sig­nificant statistical difference in pathological results (x² = 0.86, p = 0.35). An increased cysta- tin C concentration corresponds to decreased ECC values in 93%. We showed that serum cystatine C concentration can replace with great certainty the ECC method for clinical assessment of early kidney damage.