The present invention relates to a device and a method for controlling an extracorporeal blood treatment device, in particular a hemodialysis device that has a dialyzer, which is divided by a semi-permeable membrane into a blood chamber and a dialysis chamber, a blood pump for conveying blood through the blood chamber at a defined blood flowrate Qb, and a dialysis pump for conveying dialysis fluid through the dialysis chamber at a defined dialysis flowrate Qd. The control device and method according to the present invention for a hemodialysis device are based on the fact that, for different blood flow rates, in each case pre-defined during the blood treatment, the dialysis flowrates are determined at which a pre-defined clearance or dialysance is maintained with the pre-defined blood flowrates and/or that, for different dialysis flowrates in each case pre-defined during the blood treatment, the blood flowrates are determined at which the predefined clearance of dialysance is maintained.
