The FEV1-increase after inhalation of a beta 2-stimulant metered-dose aerosol was studied in 23 patients treated by specialists. The effect of their spontaneous inhalation technique was compared with that of controlled inhalation, which was optimised by means of a device controlling the breathing pattern and release of the metered-dose aerosol. This allowed quantitative assessment of the loss of bronchodilatation caused by the spontaneous inhalation technique. Thirteen patients who were observed to make inhalation errors showed a significant loss of bronchodilatation (30%), whereas ten patients who were observed to make no inhalation errors showed an insignificant loss of bronchodilatation (13%). It is concluded that when a metered-dose aerosol is used in general clinical practice there is a considerable loss of potential efficacy.