ELISA was used to investigate concentrations of Willebrand's factor antigen (WF III Ag) in 131 patients with various forms of systemic vasculitis (SV) and 93 patients with cardiovascular diseases. Compared to donors, a mean WF III Ag level proved significantly higher in all nosological variants of SV except periarteritis nodosa. Skin livedo vasculitis did not produce noticeable deviations in WF III Ag levels versus those in donors, whereas atherosclerosis obliterans showed these levels to be significantly elevated. Essential hypertension patients had low values of WF III Ag. The highest mean level of the antigen was found in rheumatoid vasculitis and Wegener's granulomatosis in which the antigen content rose to the highest for vascular affections levels (p < 0.05-0.01). High WF III Ag in Wegener's granulomatosis, periarteritis nodosa, leukocytoclastic vasculitis were indicative of renal involvement. Moreover, there was a tendency to an increase in the antigen quantities in Wegener's granulomatosis patients with intercurrent infections versus such without them. In other SV forms and atherosclerosis obliterans concentrations of the antigen were not related to clinical features of the disease.