The aim of the study was to evaluate function of endothelial cells in the patients with hyperthyrosis before thyrostatic treatment on the base of known markers of endothelial disturbances: von Willebrand factor and tissue plasminogen activator. The study was performed in the 36 patients aged 20-60 (means 36.7) years suffering from Graves disease and in 17 patients with hyperthyrotic goitre aged 30-59 (means 45.8). The control group consisted of 40 healthy volunteers. In the blood plasma concentration of tissue plasminogen activator (t-PA) and von Willebrand factor (vWF) antigen were determined with enzyme-linked immunoassay (ELISA). In patients with hyperthyroidism significantly increased antigen concentration of both studied parameters were observed. It seems that increased concentration of t-PA and vWF antigen in the blood of hyperthyrotic patients was the result of the release of them from injured thyroid gland by disease or of stimulating effect of thyroid hormones on the t-PA and vWF synthesis in the endothelial cells.