The aerosol generator of an inhalation therapy device according to a preferred embodiment of the invention continuously produces an aerosol at a generation rate of about 0.2 ml/min to about 1.5 ml/min, preferably 0.4 ml/min to about 1.2 ml/min and the mixing chamber has a volume of about 60 ml to about 150 ml, preferably about 80 ml to about 120 ml, and even more preferably about 90 to about 110 ml. While the patient inhales through the inhalation therapy device, the generated aerosol is inhaled by the patient. While the patient exhales into the inhalation therapy device, the generated aerosol is collected in the mixing chamber. Due to the specific size of the mixing chamber, the continuously generated aerosol can accumulate therein without losses even when the exhalation phase is longer than the inhalation phase like in the case of emphysema.