The use of pressurized aerosol inhalers was assessed in a total of 1173 out-patients attending hospital during a three month period. One hundred and thirty-five patients already being treated with drugs by inhalation were found to have a poor inhaler technique, even though most had received instruction on how to use a pressurized aerosol and were considered to be able to use one of these devices correctly. Eighty patients who had not used an inhaler before were found to be unable to use a pressurized aerosol efficiently after reading an instruction pamphlet or having the correct use of an aerosol demonstrated to them. Fifty-one percent of patients studied were observed to have difficulty in co-ordinating aerosol release with inspiration. In 24% the release of aerosol into the mouth caused a halt of inspiration and in a further 12% the aerosol was actuated into the mouth, but inspiration was achieved through the nose with no air being drawn in through the mouth. Twelve percent of patients already being treated with inhalers actuated the aerosol on two or more occasions during one inspiration.