Chronic obstructive pulmonary disease (COPD) is a condition characterized by airway obstruction. Patients with COPD experience significant shortness of breath on exertion. The mechanisms responsible for shortness of breath on exertion are well understood in moderate and severe COPD, but, are poorly understood in mild COPD where symptoms appear disproportionate to the degree of airway obstruction. Mild COPD patients show an exaggerated breathing response to exercise, determined by the breathing response to carbon dioxide production (V̇E/V̇CO2). Recent work suggests that the increased V̇E/V̇CO2 during exercise in mild COPD is secondary to increased deadspace (i.e. lung regions with ventilation but no perfusion) and/or ventilation/perfusion (V̇A/Q) inequality (poor matching of ventilation to perfusion). Researchers have proposed that the increased deadspace or V̇A/Q inequality is secondary to pulmonary vascular dysfunction and hypoperfusion of the pulmonary capillaries. Recently, we have shown that inhaled nitric oxide, a potent dilator of pulmonary vasculature, reduces shortness of breath and V̇E/V̇CO2, and improves exercise capacity in mild COPD. This preliminary finding suggests that pulmonary vascular dysfunction is an important contributor to exercise intolerance in mild COPD. Here, we aim to test whether sildenafil, an oral pulmonary vasodilator, can improve exercise tolerance and shortness of breath in mild COPD.