Little is known about changes in physical activity in subjects with chronic obstructive pulmonary disease (COPD) and its impact on mortality. Therefore, we aimed to study changes in physical activity in subjects with and without COPD and the impact of physical activity on mortality risk. Subjects from the Copenhagen City Heart Study with at least two consecutive examinations were selected. Each examination included a self-administered questionnaire and clinical examination. 1270 COPD subjects and 8734 subjects without COPD (forced expiratory volume in 1 s 67±18 and 91±15% predicted, respectively) were included. COPD subjects with moderate or high baseline physical activity who reported low physical activity level at follow-up had the highest hazard ratios of mortality (1.73 and 2.35, respectively; both p<0.001). In COPD subjects with low baseline physical activity, no differences were found in survival between unchanged or increased physical activity at follow-up. In addition, subjects without COPD with low physical activity at follow-up had the highest hazard ratio of mortality, irrespective of baseline physical activity level (p≤0.05). A decline to low physical activity at follow-up was associated with an increased mortality risk in subjects with and without COPD. These observational data suggest that it is important to assess and encourage physical activity in the earliest stages of COPD in order to maintain a physical activity level that is as high as possible, as this is associated with better prognosis.