A method of sensing the detachment of the venous needle (13) from a patient (P) during an extracorporeal blood treatment in a dialysis machine (1) having an extracorporeal blood circuit (2), in which an electric current is made to flow along a closed-loop path (P1) of an electrical circuit (37) formed by an electrical line (21, 22, 23) external to the extracorporeal circuit (2), a portion of extracorporeal circuit (2) directly connected to the venous needle (13), and the patient (P); a signal correlated with the electric current along the electrical line (21, 22, 23) is detected; and the signal is compared with a threshold value by means of a control unit (15).