Aim. To estimate the prevalence of autoantibodies pathognomonic for autoimmune diseases of connective tissue and liver in patients with type 1 diabetes mellitus (DM1) and to study their clinical features in patients positive for these indicators. Subjects and methods. A total of 84 patients (39 men and 45 women) with DM1 divided into 2 groups were examined. Biochemical, immunological, and instrumental examinations were performed. Results. There was a high prevalence of markers of autoimmune diseases of connective tissue and liver in patients with DM1 and that of autoantibodies in those without its clinical symptoms or signs according to instrumental findings. Conclusion. The findings may suggest that patients with DM1 have a higher risk of concomitant autoimmune diseases with a probability of their asymptomatic course.