COPD (chronic obstructive pulmonary disease) is the most common pulmonary disease and is the only common cause of death in which mortality is presently rising. It is caused by the inhalation of smoke, which leads to oxidative stress and inflammation both in the lungs and systemically. Reduced physical activity is a well-recognized consequence of the condition, but we argue here that inactivity is itself an early cause of lung function decline and symptoms. This hypothesis is supported by data from population studies that link activity levels to decline in spirometric indices, both in smokers and non-smokers. In addition, smokers with low physical activity levels are more likely to be diagnosed subsequently with COPD. Physical exercise reduces oxidative stress, has an anti-inflammatory effect and reduces the frequency of upper respiratory tract infections, providing a number of mechanisms by which it could attenuate the harmful effects of smoking. There is sufficient evidence to justify population trials of lifestyle interventions aimed at improving physical activity levels and reducing lung function decline in people diagnosed with early COPD through spirometry screening.