The objective of this work was to study the actual prevalence of diabetes mellitus complications and evaluate the quality of health care provided to the patients with these problems in different regions of Russia. A random sample of 11 240 patients with diabetes mellitus (DM) was obtained from a total of 20 regions of the Russian Federation. The study protocol included evaluation of visual function, the state of peripheral nervous system and lower extremities, renal and cardiovascular functions; in addition, HbA1c, creatinine, total cholesterol and triglyceride levels were measured along with albuminuria. It was found that the actual prevalence of major diabetic complications is 20-50% higher than the registered one. The overwhelming majority of the patients were shown to be chronically decompensated. This finding holds for 85.5% of the children, 92.6% of the adolescents, 83.9% and 74.8% of the adult patients with types 1 and 2 DM respectively. The HbA1c level in patients living in rural areas was 33.7-42.1% higher than in residents of regional urban centres. Fewer than 14.4% and 0.6% of the patients with type 2 DM used insulin and its analogs respectively. Overall, the study demonstrated rather poor quality of medical and preventive aid provided to diabetic patients and the necessity of its improvement.