The purpose of this study was to determine in a population of asthmatics, the number of patients who correctly used the pressurized aerosols (bronchodilator drugs or steroids) spontaneously, or after training sessions. 115 asthmatics (in- or outpatients, 14-70 years old) were asked to use their inhaler in the presence of an observer. The inhalation technique was considered correct when (1) the puff release was coordinated with a deep inspiration and (2) when the inspiration was followed by a few seconds' breath-holding. The patients were divided into two groups: a trained group of 46 patients who were observed for 1 month or more after the two above-mentioned maneuvres had been demonstrated to them by a physician and the need for correct use strongly emphasized. The other group (the untrained group) comprised 69 patients who were presumably using their inhalers according to the manufacturer's instructions: (1) expiration; (2) inspiration and actuation, and (3) apnea. In the trained group 52% of the patients were classified by the observer as correct users, as compared to 28% in the untrained group. The results were not influenced by sex. age or occupational differences. These observations suggest that a majority of asthmatics probably derived incomplete benefit from the use of pressurized aerosols. Although training apparently results in a more efficient use of the canisters, this study shows that training sessions must be repeated, and the results checked at regular intervals by a member of the medical staff. In subjects who repeatedly fail to achieve a correct inhalation technique the drug should be given by another route.