['To investigate whether the bronchodilatory effect of deep inspiration is impaired in subjects with COPD.', 'We measured deep inspiration-induced bronchodilation in 19 patients with COPD and 17 healthy subjects (mean age, 67.8 +/- 7.1 years vs 62.5 +/- 9.3 years, respectively [+/- SEM]). Each subject underwent a series of single-dose methacholine provocations to induce at least a 15% reduction in inspiratory vital capacity (IVC). When this was achieved, subjects were asked to perform four consecutive deep inspirations, after which the IVC measurement was repeated and the percentage of bronchodilation by deep inspiration was calculated.', 'The percentage of reduction in IVC from baseline prior to the deep inspirations did not differ between the two groups (COPD, 20.1 +/- 1.6%; healthy, 22.7 +/- 2.4%; p = 0.38); median single methacholine doses employed were 20 mg/mL (range, 0.025 to 75 mg/mL) and 25 mg/mL (range, 10 to 75 mg/mL), respectively (p = 0.19). Deep inspirations were not able to reverse bronchoconstriction in patients with COPD (bronchodilation, 3.9 +/- 2.6%; p = 0.15 by one-sample t test). Bronchodilation by deep inspiration was present in healthy subjects (13.7 +/- 3.0%, p = 0.0003), and was significantly higher than that of patient with COPD (p = 0.02). In patients with COPD, deep inspiration-induced bronchodilation correlated with the percentage of predicted transfer factor of the lung for carbon monoxide (r = 0.53, p = 0.05), but not with airway obstruction, as assessed by FEV(1) or FEV(1)/FVC.'] Is bronchodilatory effect of deep inspiration absent in subjects with mild COPD?