Bronchodilator response is usually tested to establish reversibility of airflow obstruction, to aid in diagnosis, and to help plan long-term bronchodilator therapy. Among the various tests to establish bronchodilator response, FEV1, FEF25-75% or FEF50%, and FVC are the most widely used. However, it may be difficult to interpret changes in pulmonary function in COPD patients because of day-to-day variability. A 15 per cent increase in FEV1 or FVC and a 30 per cent increase in isovolume FEF25-75% or FEF50% above baseline are acceptable criteria for bronchodilator response. beta-Adrenergic agonists, theophylline, and anticholinergic agents are available for testing bronchodilator response. Inhalation of two puffs of a beta-adrenergic agonist aerosol from an MDI is the most convenient and practical approach for testing in the laboratory. Bronchodilator response is presumed to be beneficial in the treatment of asthma. However, in the treatment of COPD a positive response may not be a reliable guide for establishing long-term benefit.