Aim To evaluate the endothelial function using an Endo-PAT2000 instrument before and after mental stress tests in patients with Tacotsubo syndrome (TS) during acute and long-term periods and to compare the obtained results with laboratory markers of endothelial dysfunction (ED).Material and methods This study included 45 patients with TS (mean age, 63.5±13.7 years) and 40 healthy volunteers (control group, CG). All patients of the main group during the acute period (first 7-14 days) and long-term period (at 1 and 2 years), as well as CG subjects, underwent evaluation of the endothelial function with an Endo-PAT 2000 instrument, and the reactive hyperemia index (RHI) was determined before and after mental stress tests. Also, concentrations of endothelin 1 (ET-1) and numbers of circulating endothelial cells (CEC) were measured after a two-year follow-up of TS patients.Results During the acute period of disease, all TS patients (n=45) had ED: RHI was below the threshold level of 1.67; furthermore, 42 (93.3 %) patients retained a lower RHI following mental stress. At one year (n=40), 16 (40%) patients showed ED at rest along with a significantly increased mean RHI. Mental stress produced at one year was associated with ED in 28 (70 %) TS patients. At two years (n=44), resting RHI was lower than normal in 19 (43.2 %) patients. Mental stress tests performed at two years were associated with ED in 29 (65.9 %) patients (RHI ≤1.67). Only 10% of CG subjects had a lower-than-normal RHI, which was significantly less than in the main group of TS patients during the acute and long-term periods (p<0.05). Mean values of laboratory markers for ED also were significantly different between TS patients during the long-term period of disease (n=41) and CG subjects (n=40; p<0.01).Conclusion During acute and long-term periods of disease, most of TS patients had impaired vascular reactivity both at rest and during mental stress. The laboratory markers of ED, ET-1 and CEC can be used in clinical practice for evaluation of the risk for TS.