Background. Type 1 diabetes mellitus (DM 1) disrupts all types of metabolism, especially carbohydrate and lipid ones, leading to many complications. The most serious DM complication is damage to the cardiovascular system (CVS). Prolonged preclinical changes precede the development of pathological processes in the CVS. One of the earliest manifestations of cardiovascular system dysfunction is myocardial remodeling. In patients with DM 1, myocardial remodeling is diagnosed using echocardiography. However, the examination algorithm does not include echocardiography for young patients with DM 1 without evident cardiovascular diseases (CVD). The state of the heart's extracellular matrix and the presence of fibrotic changes in it are actively studied as a marker of changes in the CVS at the preclinical stage. Assessment of the presence of fibrosis foci is available during echocardiography; however, it is generally relevant for patients with DM 1 with pre-existing CVD. The initial formation of diffuse fibrosis (DF) of the myocardium in patients with DM without diagnosed CVD can be verified by magnetic resonance imaging (MRI) of the heart. Given that DF increases the risk of life-threatening arrhythmias and significantly increases the risk of sudden cardiac death in young patients with DM 1, it is necessary to revise the strategy of primary prevention of CVD for patients in this population. Aim. To assess the CVS structural and functional state in young patients with DM 1 without CVD according to echocardiography and MRI of the heart using T1 mapping. To determine the presence and instrumental imaging options of fibrous tissue in the myocardium without laboratory markers of its formation in young DM 1 patients without CVD. Materials and methods. The study included 110 participants without CVD and obesity: 80 patients with DM 1 (mean age 26 years) and 30 patients in the comparison group without DM (mean age 27 years). All participants underwent a general clinical examination, bioelectrical impedance analysis, electrocardiography, echocardiography, and cardiac MRI with T1 mapping. Conclusion. DF of the myocardium was detected using heart MRI in 8.7% of young patients with DM 1. In healthy peers, these changes in the myocardium were not detected. The group of patients with DM 1 and DF of the myocardium had a longer duration of the disease (8 years vs 5 years) and more severe initial structural changes in the myocardium (according to echocardiography) compared to patients with DM 1 without myocardial fibrosis. In the presence of initial signs of myocardial remodeling in patients with DM 1, it is advisable to conduct a heart MRI to exclude the DF formation.