Objective. To study a role of long-term stress and endothelial function in the development of autonomic dystonia syndrome (ADS) in young patients. Material and methods. Authors examined 75 male patients, aged from 17 to 28 years, with ADS including 41 patients without clinically evident symptoms of psycho-emotional stress (group I) and 34 patients with high level of psycho-emotional stress (group II). A comparison group consisted of 21 healthy volunteers. The level of anxiety was assessed using the Spilberger—Khanin Inventory and the functional state of endothelium with the use of endothelium-dependent vasodilatation (EDV). Standard bicycle physical exercise (PE) test was performed. Results. Correlations between state and trait anxiety in all groups were shown. In group II, these values were maximal thus demonstrating marked anxiety. In EDV and PE tests, patients of group I had general reactivity of the vascular wall, while patients of group II showed opposite reactions, either increase or decrease in the brachial artery diameter. This instability may result from the increased liability of the sympathetic part of the autonomic nervous system in stress conditions. Conclusions. Long-term stress causes vascular endothelial dysfunction in young patients with ADS that increases the severity of clinical manifestations of the disease and leads to the increased risk of cardiovascular pathology.