The invention relates to a method and to a device for determining an optimum dialysate flow Qdopt for an extracorporeal blood treatment and to a blood treatment device comprising a device 18 for determining an optimum dialysate flow Qdopt. The optimum dialysate flow Qdopt is determined on the basis of a relationship describing the dependence of the clearance K on the dialysate flow Qd. The device according to the invention comprises a measurement device 18B for measuring at least one value which is characteristic of the clearance K, a calculation and/or evaluation unit 18A of the device according to the invention being configured in such a way that the clearance K is determined on the basis of the at least one value which is characteristic of the clearance. The calculation and/or evaluation unit 18A is configured in such a way that the optimum dialysate flow Qdopt is determined from the relationship describing the dependence of the clearance K on the dialysate rate Qd on the basis of the measured clearance K, or the optimum dialysate flow Qdopt is determined from the measured clearance K.
