A device for monitoring the access to the cardiovascular system of a patient undergoing an extracorporeal treatment of blood in a machine (1) comprising a treatment device (4) and an extracorporeal circuit (2), comprises: a voltage generator (16) for generating a potential difference between a part of the machine (1) and a first point (B) of a venous branch (8) of the extracorporeal circuit (2), connecting the patient to the treatment device (4); a detector (17) for detecting the value (dV) of a quantity that correlates with the electric current along at least one section (10a; 10b; 10c) of the venous branch (10) between the first point (B) and a venous needle (13) fitted at the end of the venous branch (8) and inserted in the vascular system of the patient (P); calculating means (15) for comparing the detected value (dV) with a reference range (T).