The acute bronchodilator effect of aerosolized fenoterol, salbutamol, ipratropium bromide, and terbutaline was tested in single-blind cross-over trials by whole body plethysmography. In 4 different series, patients with chronic obstructive airway disease were investigated using combined alternating medication. Only commercially available metered dose inhalers were included in this study. Special attention was paid to comparison of the efficacy of administration by the investigator and by the patients themselves. The doses recommended by the manufacturers sometimes resulted in widely different bronchodilation when the metered dose inhalers were administered by the patients. Some possible reasons for these results and some implications with regard to different types of inhalers, single doses, instruction of the patients and evaluation of clinical trials are discussed.