The invention relates to a blood treatment device having an extracorporealbloodcircuit which comprises an arterial line, a blood pump, a blood treatment unitand avenous line, wherein the arterial and venous lines can be connected to a bloodvessel of a patient, and wherein the blood treatment device has an evaluationandcontrol unit, wherein the evaluation and control unit is configured to carryout thefollowing steps: (a1) determining the blood recirculation in a blood vessel ofthepatient connected to the extracorporeal blood circuit； and (b) calculating thebloodflow in the blood vessel using the blood recirculation determined inaccordance with(a1) and using a provided value or a value likewise previously determined forthecardiac output of the patient.
