1. We measured changes in peak expiratory flow rate (PEFR), forced expiratory volume in 1 s (FEV1·0), airways resistance (Raw), specific conductance (sGaw), residual volume (RV), functional residual capacity (FRC) and total lung capacity (TLC) in 44 patients with asthma.
2. When asthma was induced by exercise in five patients there were large changes in volumes, but these did not obscure changes in PEFR, which adequately defined the time course of the response.
3. In 70 comparisons before and after inhalation of bronchodilator drug in 33 asthmatic subjects, the responses were classified by the size of the change in lung volumes, which showed a concordant improvement, or no change, in 61 comparisons. Despite these lung volume changes, measurement of both PEFR and FEV1·0, would have detected a bronchodilator response in all but two cases.
4. In 81 comparisons in 23 subjects over time intervals varying from 1 day to 11 months, lung volumes changed in concordance with PEFR and FEV1·0 in 59. In eight of these comparisons, measurement of lung volumes would have altered our interpretation of the changes in PEFR and FEV1·0.
5. In the same 81 comparisons changes in airways resistance were concordant with changes in PEFR and FEV1·0 on 44 occasions, with minor discordant changes in 19. We could not explain the remaining 18 cases showing major discordance between these two types of measurement of airway calibre.
6. We conclude that both FEV1·0, and PEFR should be used for detection of a bronchodilator response, and that measurement of lung volumes will rarely contribute to the interpretation. Over longer periods, lung volumes should be measured if possible. We found no practical use for routine measurement of airways resistance in patients with asthma.