A method and a device for determining the blood flow Q.sub.F in a vascular access (F) during an extracorporeal blood treatment is described, where the blood enters the blood treatment unit (3) of the blood treatment machine through an arterial branch (19) of an extracorporeal circulation loop (2) which is in fluid connection with the vascular access at an arterial connection. The blood is returned through a venous branch (21) of the extracorporeal circulation, which is in fluid connection with the vascular access at a venous connection (13). The blood flow in the vascular access is determined by measuring the pressure p.sub.art, p.sub.ven, in the arterial and/or venous branch of the extracorporeal circulation when the vascular access is open and interrupted, while the extracorporeal blood flow Q.sub.B is varied. Then the fistula flow Q.sub.F is determined from the measured values for the arterial and/or venous pressure while the vascular access is open and interrupted.