Extracorporeal blood treatment device for carrying out an extracorporeal blood treatment, in which blood flows in an extracorporeal blood circuit through the blood chamber of a dialyser which is separated by a semi-permeable membrane into the blood chamber and a dialysate chamber, the blood treatment device comprising: a fluid system including the dialysate chamber at least during operation, a pump system for filling the fluid system including the dialysate chamber with a liquid and for establishing a pressure difference between the blood chamber and the dialysate chamber, the pump system (6, 17, 24) having at least one pump, a pressure measuring apparatus for measuring the pressure in the fluid system including the dialysate chamber for carrying out an integrity test for monitoring the integrity of the dialyser, a control and arithmetic unit which cooperates with the pump system and the pressure measuring apparatus and which provides a preparation mode in preparation for the blood treatment, characterised in that, for carrying out the preparation mode, the control and arithmetic unit is so configured that the fluid system including the dialysate chamber is filled with a liquid, the integrity test for monitoring the integrity of the dialyser is carried out with a semipermeable membrane wetted only on the dialysate side after the fluid system including the dialysate chamber has been filled with a liquid, and the blood chamber is filled with a liquid after the integrity test has been carried out, the control and arithmetic unit being so configured for carrying out the integrity test in the preparation mode that a pressure difference is established between the blood chamber and the dialysate chamber in such a manner that the pressure in the blood chamber is greater than the pressure in the dialysate chamber, the increase in the pressure p, measured by the pressure measuring apparatus, in a given time interval being determined, and a leakage rate LR being determined from the increase in the pressure in a given time interval and the compliance C of the fluid system including the dialysate chamber, and a lack of integrity of the dialyser being concluded on the basis of the determined leakage rate LR. a fluid system including the dialysate chamber at least during operation, a pump system for filling the fluid system including the dialysate chamber with a liquid and for establishing a pressure difference between the blood chamber and the dialysate chamber, the pump system (6, 17, 24) having at least one pump, a pressure measuring apparatus for measuring the pressure in the fluid system including the dialysate chamber for carrying out an integrity test for monitoring the integrity of the dialyser, a control and arithmetic unit which cooperates with the pump system and the pressure measuring apparatus and which provides a preparation mode in preparation for the blood treatment, characterised in that, for carrying out the preparation mode, the control and arithmetic unit is so configured that the fluid system including the dialysate chamber is filled with a liquid, the integrity test for monitoring the integrity of the dialyser is carried out with a semipermeable membrane wetted only on the dialysate side after the fluid system including the dialysate chamber has been filled with a liquid, and the blood chamber is filled with a liquid after the integrity test has been carried out, the control and arithmetic unit being so configured for carrying out the integrity test in the preparation mode that a pressure difference is established between the blood chamber and the dialysate chamber in such a manner that the pressure in the blood chamber is greater than the pressure in the dialysate chamber, the increase in the pressure p, measured by the pressure measuring apparatus, in a given time interval being determined, and a leakage rate LR being determined from the increase in the pressure in a given time interval and the compliance C of the fluid system including the dialysate chamber, and a lack of integrity of the dialyser being concluded on the basis of the determined leakage rate LR.