Introduction. The most common complication of the vascular access for hemodialysis, that can lead to its loss, is thrombosis. The method of choice in diagnosing fistula thrombosis is duplex ultrasound. Purpose: to study the ultrasound signs and changes in hemodynamics data in thrombosed fistulas and to determine the risk factors contributing to its development. Materials and methods. Duplex ultrasound was performed in 550 patients with vascular access for hemodialysis. Access thrombosis was detected in 12,0%, non-occlusive thrombosis was observed in 60,6%, occlusive thrombosis — in 39,4%. Thrombosis was significantly more common in women than in men (p=0,025). In patients with graft, thrombosis was more common (24,2%) than in those with arteriovenous fistula (11,2%), p=0,026. A correlation was found between the presence of significant vascular access stenosis, aneurysm of the outflow vein or graft, and the development of thrombosis (p<0,02). Duplex ultrasound allows to diagnose vascular access thrombosis and to determine the causes of its development.