The risk of devolvement of type 1 diabetes mellitus (DM1) remains a challenging problem because neither etiology of the disease nor its prognosis and genetic predisposition to this condition are clearly understood. The development of any autoimmune process starts from the disturbance of subtle molecular mechanisms involved in the regulation of the immune system. Therefore, the genes controlling the function of its major components are at the same time the potential candidate genes encoding for the predisposition to DM1. Their association with the disease was studied by means of comparative analysis of the frequency distribution of alleles and genotypes of the polymorphous rs3087243 (G6230A) marker of the CTLA4 gene encoding for antigen-4 of cytotoxic T-lymphocytes. The present study included 257 patients presenting with type 1 diabetes mellitus and 526 healthy subjects. Genotypes were identified by the "real time" amplification technique. The AA genotype was found to occur less frequently in the diabetic patients than in the control group (11.3% and 22.1% respectively). In contrast, the frequency of the GG genotype was higher in the patients with DM1 than in the healthy subjects (44.7% and 37.5% respectively). It is concluded that the polymorphous rs3087243 marker of the CTLA4 gene is significantly associated with the predisposition to the development of type 1 diabetes mellitus in the patients of Russian descent.