Rationale: Although the major limitation to exercise performance in COPD patients is dynamic hyperinflation, little is known about its relation to daily physical activity. Objectives: To analyze the contribution of dynamic hyperinflation, exercise tolerance and airway oxidative stress to physical activity in COPD patients. Methods: In a cross-sectional study, we included 110 patients with moderate to very severe COPD. Daily physical activity was measured using a triaxial accelerometer providing a mean of 1-minute movement epochs as vector magnitude units (VMU). Patients performed the six-minute walk test, incremental exercise test with measurement of breathing pattern and operating lung volumes, and constant-work rate test at 75% of maximal work rate. Measurements and Main Results: GOLD stage and BODE index, arterial blood gases, lung volumes, diffusing capacity and biomarkers in exhaled breath condensate (EBC). Daily physical activity was lower in the 89 patients who developed dynamic hyperinflation than in the remaining 21 who did not [161 (SD 70) vs. 288 (SD 85) VMU;P=0.001]. Physical activity was mainly related to distance walked in six minutes (r=0.72;P=0.001), peak oxygen uptake (r=0.63;P=0.001), change in end-expiratory lung volume during exercise (r=-0.73;P=0.001), endurance time (r=0.61;P=0.001) and 8-isoprostane in EBC (r=-0.67;P=0.001). In a multivariate linear regression analysis using VMU as a dependent variable, dynamic hyperinflation, change in end-expiratory lung volume and distance walked in six minutes were retained in the prediction model (r²=0.84; P=0.001). Conclusions: Daily physical activity of COPD patients is mainly associated with dynamic hyperinflation, regardless of severity classification.