The invention relates to a method and a device for monitoring a vascular access during an extracorporeal blood treatment. The method and the device according to the invention are based on the monitoring of the difference between the venous pressure measured by means of a venous pressure sensor (19) and the arterial pressure measured by means of an arterial pressure sensor (18) in the extracorporeal blood circuit, said pressure being referred to as venous and arterial differential pressure. The invention is based on the finding that the values measured for the venous and arterial pressure are superimposed by interfering signals which in general make it difficult to reliably evaluate the measured pressure signals for identifying a defective vascular access on the basis of the differential pressure only. According to the method and the device according to the invention, a test function describing disturbances in the extracorporeal blood circuit is determined. Said test function is used to determine a noise-free differential pressure from the measured venous and arterial pressure, said differential pressure being evaluated in an arithmetic and evaluation unit (22) to identify a defective vascular access.
