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 a venous pressure sensor and the arterial pressure measured by an arterial pressure sensor (in the extracorporeal blood circuit. 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 to identify a defective vascular access.