BACKGROUND: Patients with type 1 diabetes mellitus (T1D) have an earlier age of onset and a longer course of the disease, already by middle age they have the development of microand macrovascular diabetic complications that reduce the quality and duration of life.AIM: To evaluate the prevalence of chronic kidney disease (CKD) and other late complications of T1D depending on renal dysfunction in the population of patients with T1D with disease duration of 20 and more years, who underwent examination and treatment in Endocrinology Research Centre.MATERIALS AND METHODS: A one-stage single-center epidemiological non-randomised study was conducted using the database of Endocrinology Research Centre with the study of 500 patients’medical histories with long-term T1D (20 years and mores), without kidney damage and with CKD at different stages (CKD C1–C5, C5D, after transplantation), examined and treated from 2011 to 2023.RESULTS: Normal renal function was observed in 10.8% of patients (n=54). Terminal stage of CKD was reached in 28.0% (n=140), of which 12.4% were on renal replacement therapy with program hemodialysis (RRT-HD), and 12.0% after isolated kidney transplantation or combined kidney and pancreas transplantation, the rest were at different stages of CKD. Normoalbuminuria was observed in 15.4% (n=77) among 500 patients. The prevalence of late complications of DM among the examined patients was high and increasing with the progression of renal dysfunction: diabetic retinopathy was diagnosed in 96% of patients, distal symmetrical polyneuropathy — in 97% of patients, various forms of autonomic neuropathy — in more than half of patients. About 60% of patients had diagnosed arterial atherosclerosis in the legs, about one third — atherosclerosis of brachiocephalic arteries, 23% — confirmed coronary heart disease, and suffered cardiovascular events (myocardial infarction, acute cerebral circulation disorder) — 19% of patients, about half of whom had CKD of different severity. Factors for increased risk of cardiovascular disease: estimated glomerular filtration rate (eGFR)<60 mL/min/1.73m^2, OR=7.1; 95% CI 3.6–8.4; p<0.001), eGFR <30 mL/min/1.73m² OR=8.7; 95% CI 2.8–8.4; p<0.001), eGFR <15 mL/min/1.73m² OR=14; 95% CI 6.3–31.3; p<0.001); albuminuria > 30 mg/g OR=2.4; 95% CI 1.6–3.6; p<0.001), dialysis OR=14.1; 95% CI 6.2–32.1; p<0.001), kidney transplant OR=11.7; 95% CI 5.4–24.9; p<0.001). Manifestation of T1D between 1996–2002 reduced the risk of developing CKD by 10.75; 95% CI 4.37; 27.03) vs manifestation of T1D earlier. Age of T1D debut 6–17 years increased the risk of reaching terminal CKD vs age of debut >18 years: OR=2.4; 95% CI 1.22; 5.022; p=0.012).CONCLUSION: Despite a significant reduction in the risk of developing CKD in individuals with T1D debut between 1996 and 2002, renal dysfunction is a frequent complication in patients with a long disease course, combining with other late complications and contributing to a high risk of terminal stage of CKD and cardiovascular events. Early age of T1D debut increases the risk of terminal CKD.