Pulmonary hypertension is a common complication of chronic obstructive pulmonary disease (COPD). The increase in pulmonary artery pressures is often mild to moderate. However, 5-10% of patients with advanced COPD may suffer from severe pulmonary hypertension and present with a progressively downhill clinical course because of right heart failure added to ventilatory handicap. The prevalence of clinically significant severe pulmonary hypertension in COPD is roughly estimated to be of 1-2/1,000. The cause of pulmonary hypertension in COPD is generally assumed to be hypoxic pulmonary vasoconstriction leading to permanent medial hypertrophy. However, recent pathologic studies point rather at extensive remodeling of all layers of the pulmonary arterial walls. These aspects account for minimal reversibility with supplemental oxygen. There may be a case for pharmacologic treatment of pulmonary hypertension in selected patients with advanced COPD and right heart failure. However, it will be a challenge for randomized controlled trials to overcome the difficulties of the diagnosis of right ventricular failure and the definition of a relevant primary endpoint in pulmonary hypertensive patients with COPD.