The invention relates to a device and a method for monitoring an access to a patient, in particular a vascular access in extracorporeal blood treatment, in which a patient's blood is withdrawn from the patient via an arterial hose line (6) and is returned to the patient via a venous hose line (7). In the device according to the invention and in the method according to the invention, an alternating voltage signal, referred to a common ground potential, is coupled in and out of the arterial and venous hose lines (6, 7), and the blood flowing through the arterial and venous hose lines is at ground potential. In this way, disturbances, which can be attributed particularly to movements of the hose lines, are reduced.
