Objective: to assess the clinical significance of cystatin C in the early diagnosis of diabetic nephropathy Materials and methods. We examined 499 patients with type 1 and type 2 diabetes mellitus older than 25 years.The laboratory examination of the patients included the measurement of cystatin, creatinine in the blood serum, GFR calculation according to the CKD-EPI equation, and the determination of albuminuria levels. Results. The role of cystatin C as a reliable marker of diabetic nephropathy has been demonstrated: the level of cystatin C ≥ 0.72 mg/L with sensitivity of 90.8 %, specificity of 88.9 % was associated with a decline of renal function in diabetic patients (ROC AUC = 0.951). Conclusion. The determination of the level of cystatin C, alone or in conjunction with creatinine, will help to more accurately diagnose GFR and assess the risk of mortality and renal complications in patients with diabetes mellitus.