The present invention relates to a method for checking a dialyzer for the presence of a leak in the semipermeable membrane of the dialyzer, wherein the membrane divides the inner dialyzer space into a least one blood chamber and into at least one dialyzate chamber, wherein the blood chamber is flowed through by blood in the operation of the dialyzer and is in fluid communication with a blood-side line system and the vascular system of the patient, and wherein the dialyzate chamber is flowed through by dialysis fluid in the operation of the dialyzer and is in fluid communication with a dialyzate-side line system, wherein the method comprises the following steps: a) emptying the blood chamber or the dialyzate chamber of blood and of dialysis fluid respectively and keeping the fluid (blood or dialyzate) in the non-emptied dialyzate chamber or blood chamber; b) building up a test pressure by means of a gas, in particular by means of air, in the emptied blood chamber or in the emptied dialyzate chamber; and c) measuring the pressure drop over time in the emptied blood chamber or in the emptied dialyzate chamber or in the line system respectively in fluid communication therewith and/or measuring the pressure increase in the non-emptied blood chamber or in the non-emptied dialyzate chamber or in the line system respectively in fluid communication therewith or measuring the number of air bubbles or of a parameter correlated with the number of air bubbles in the non-emptied blood chamber or in the non-emptied dialyzate chamber or in a line system respectively in fluid communication therewith, wherein the steps a) to c) are carried out subsequent to the blood treatment of the patient and subsequent to the disconnection of the patient from the blood-side line system.