Many different metered-dose inhalation devices are becoming available for the treatment of airway diseases. Each of these inhalers differs in its delivery characteristics. An assessment of the efficacy of drug delivery by these inhalers is essential, in view of their therapeutic use. A review of the literature on the relationship between airway deposition and airway effects of drugs delivered from metered-dose inhalers is presented. Nebulizers or spacers are not discussed. The effect of an inhaler depends on the characteristics of the inhaler and the inhalation manoeuvre performed by the patient. This review focuses on the influence of inhaler characteristics on the airway deposition and airway effects. Data from several studies show that there is a significant relationship between the amount of drug deposited in the airways and the airway effects of the drug. Studies on the relationship between airway deposition and airway effect have been troubled by methodological problems, such as the absence of multiple dose comparisons and the difficulty in obtaining steep dose-response curves. The techniques for measuring airway deposition of inhaled drugs, namely the scintigraphic and the pharmacokinetic method, are discussed and compared. The appropriate use of these techniques can help to define and compare the drug delivery characteristics of different devices, thus enabling inhaled therapy to be optimized.