Cystatin C is a low molecular weight protein which can be used as a marker of glomerular filtration rate (GFR). By comparing GFR (determined with 51Cr-EDTA) of cystatin C and creatinine, cystatin C was found to be superior to creatinine as an indicator of GFR since the serum creatinine concentration changes significantly only when GFR is already impaired by 50%. Cystatin C serum concentration is mainly determined by GFR. Combined measurement of cystatin C serum concentration and its excretion in urine is useful in assessing early decrease of GFR and proximal tubule damage. The aim of this study was to assess the influence of proteinuria on cystatin C serum concentration. We determined the serum cystatin C concentration, 24h protein excretion in urine, serum urea, creatinine, total proteins and albumin concentration, plasma colloid osmotic pressure (COP) and endogenous creatinine clearance. The study included 45 patients (28 males and 17 females), average age 39.51 ± 7.60 years, with average endogenous creatinine clearance 97.70 ± 12.08 mL/min. Patients were divided into three groups according to the level of glomerular proteinuria. The first, control group, included 15 persons (6 males and 9 females, average age 37.27 ± 5.48 years) with proteinuria level <0.25 g/24h and average endogenous creatinine clearance 99.70 ± 12.94 mL/min. Patients with primary glomerulonephritis (PGN) in the second group (11 males and 4 females, average age 38.93 ± 8.32 years) had proteinuria range 0.25-3.5 g/24h and endogenous creatinine clearance 94.00 ± 11.10 mL/min. The third group of patients with nephrotic proteinuria (>3.5 g/24h) included 15 patients (11 males and 4 females, average age 42.33-8.23) with PGN and endogenous creatinine clearance 99.40-12.07 mL/min. Results were statistically analysed with Student t-test and Mann-Whitney est, with levels of significance 0.05 and 0.01. Proteinuria had no significant influence on cystatin C serum concentration in patients with primary glomerulonephritis and endogenous creatinine clearance over 80 mL/min. Cystatin C is an adequate indicator of glomerular filtration rate in proteinuric patients.