The invention relates to a method for checking a dialyzer for the presence of a leak in the semipermeable membrane of the dialyzer. The membrane divides the dialyzer interior into at least one blood chamber and at least one dialysate chamber. Blood flows through the blood chamber during the operation of the dialyzer, and the blood chamber is fluidically connected to a blood-side line system and the vascular system of the patient. Dialysis fluid flows through the dialysate chamber during the operation of the dialyzer, and the dialysate chamber is fluidically connected to a dialysate-side line system. The method has the following steps: a) emptying the blood chamber or the dialysate chamber, b) building up a test pressure, and c) measuring the pressure decrease and/or measuring the pressure increase, wherein steps a) to c) are carried out following the blood treatment of the patient and following the disconnection of the patient from the blood-side line system.
