The present invention relates to a method of compensating the pressure drop in an ordinary ventilator tube. The method comprises the measurement of a first air flow at a first pressure on the apparatus' side of the ventilator tube and with the respiratory mask taken off. Moreover, a parameter is calculated in a function which provides the pressure drop in the ventilator tube in dependence on the air flow. The parameter for the air flow and pressure is calculated. Later, the air flow is calculated with the respiratory mask being put on. By means of the function a correction pressure is calculated from the air flow. Finally, a target pressure is set which results as a sum of the selected mask pressure and the correction pressure. The invention moreover relates to ventilator for performing the aforementioned method and to a memory medium for storing a corresponding program.