Although the manufacturers of pressurized aerosol bronchodilators issue instructions for using the inhalers, little or no experimental verification exists. Furthermore, the instructions often fail to take into account known facts about aerosol deposition. We have reviewed the evidence for the most effective mode or modes of inhalation of pressurized aerosol bronchodilators, some of which has arisen from experiments performed in our laboratory. In order to achieve a maximal effect following inhalation of terbutaline sulphate bronchodilator aerosol (Bricanyl, Astra Pharmaceuticals), the canister should be actuated during a slow (25 l min-1), deep inhalation and breath held subsequently for 10 s. Bronchodilatation may be reduced if aerosol is inhaled rapidly (80 l min-1) or if breath is held for 4 s. We have considered the applicability of these simple rules to other types of bronchodilator and to all patients, irrespective of their diagnosis or degree of airway obstruction. The lung volume at which aerosol is released into the airstream, the importance of coordinating inhalation with aerosol actuation, and the relative merits of "open" and "closed" mouth inhalation techniques are also discussed.