The paper is concerned with clinical analysis of 1856 case records of patients with diffuse toxic goiter (DTG) to determine the frequency of development of diabetes mellitus in this pathology. In order to study glucose homeostasis and pancreatic beta-cell function, the blood levels of glucose and IRI were investigated in 107 DTG patients (in decompensation stage) on an empty stomach and 30, 60 and 120 min. after glucose testing. Diabetes mellitus was noted in 8.6% of the DTG patients, i. e. thrice as more frequent as in the general population. Glucose tolerance disorders were revealed more than in a half of the patients (in 55% of the examinees), 19% of them had latent or evident diabetes mellitus. Pathogenesis of glucose homeostatic disorders was closely associated with function of the insular apparatus. Relative and then absolute insufficiency of beta-cells with respective changes develop gradually against a background of their elevated function at the onset of disease. The frequency and a degree of glucose tolerance disorders and beta-cell dysfunction were in direct relation to the gravity of a clinical course of DTG.