Patients with severe chronic obstructive pulmonary disease (COPD) develop dynamic lung hyperinflation (DH) during symptom-limited incremental and constant work exercise with cycle ergometer and treadmill. The increase in end-expiratory lung volume seems to be the best predictor of dyspnea. Quantification of DH is based on the relatively complex use of on-line measurement of inspiratory capacity (IC) from flow volume loops. We reasoned that DH could occur during daily activities such as walking, and that it could be simply measured using the spirometrically determined IC. We studied 72 men with COPD (FEV(1) = 45 +/- 13.3% predicted). IC was measured at rest and after a 6-min walk test. Exertional dyspnea was evaluated using the Borg scale and dyspnea during daily activities with the modified Medical Research Council (MRC) scale. IC decreased significantly from 28.9 +/- 6.7% TLC at rest to 24.1 +/- 6.8% TLC after exercise (p < 0.001). Exertional dyspnea correlated with DeltaIC (r = -0.49, p < 0.00001) and baseline MRC (r = 0.59, p < 0.00001). In many patients with COPD, walking leads to DH that can be easily determined with simple spirometric testing. DH helps explain exercise capacity limitation and breathlessness during simple daily activities.