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Chronic obstructive pulmonary disease (COPD) is a systemic disease with various outcomes of importance, for example, exercise capacity and dyspnea. These variables may follow different trajectories over time. It is established that physical activity decreases in COPD, although its process over time has not been evaluated. Accordingly, we compared longitudinal changes in physical activity with changes in standard outcome assessments: forced expiratory volume in 1 second (FEV1), 6-minute walk distance, and dyspnea.Physical activity was measured with tri-axial accelerometers worn on a lateral position at the waist for 7 consecutive days. Nonuse was eliminated using a computer algorithm for this device. Activity was assessed in 2 ways: (1) as vector magnitude units (VMU), the sum of movements per minute in 3 planes, and (2) VM250, the percent of time worn when VMU was 250 or more counts per minute. Eighteen patients with COPD (FEV1 61 ± 17% predicted) from a larger population were restudied. The interval between the baseline and followup assessments was 609 ± 58 days.Mean VMU at followup was less than at baseline: 146 ± 70 vs 198 ± 85 counts per minute, respectively (P = .002). VM250 was also reduced: 19 ± 9% vs 25 ± 11%, respectively (P = .003). There was no significant longitudinal change in FEV1. The 6-minute walk distance decreased by 39 m (P = .04). The Medical Research Council dyspnea increased by 0.5 units (P = .04).Directly measured physical activity decreases over time in COPD, similar to other outcomes of importance.