Patients with types I and II diabetes mellitus without the clinical signs of nephroangiopathy were subjected to a comprehensive study of renal function. Using the method for determination of endogenous creatinine clearance (140 patients), radionuclide methods (in 350 patients) and beta-2-microtest (in 130 patients) the impairment of renal filtration function was established in 30.8, 67.6 and in 52% of the patients, respectively. In 16.3% of the patients, there was an impairment of the proximal part of the canaliculi. The determination of proteinuria selectivity in polyacrylamide gel revealed latent proteinuria in 70% of the patients and enabled one to specify the spectrum of uroproteins. The informative value of the methods suggested was supported by the results of morphologic studies of biopsy specimens of the kidneys and skin. The importance of the specification of the preclinical stage of diabetic nephroangiopathy was determined by the possibility of the reverse development of the pathological alterations at the given stage. The authors describe the experience gained with the correction of functional disorders by means of programmed administration of insulin using the apparatus Biostator, application of diamicron and angioprotectors.