Mild-to-moderate pulmonary hypertension is a common complication of chronic obstructive pulmonary disease (COPD); such a complication is associated with increased risks of exacerbation and decreased survival. Pulmonary hypertension usually worsens during exercise, sleep and exacerbation. Pulmonary vascular remodelling in COPD is the main cause of increase in pulmonary artery pressure and is thought to result from the combined effects of hypoxia, inflammation and loss of capillaries in severe emphysema. A small proportion of COPD patients may present with "out-of-proportion" pulmonary hypertension, defined by a mean pulmonary artery pressure >35-40 mmHg (normal is no more than 20 mmHg) and a relatively preserved lung function (with low to normal arterial carbon dioxide tension) that cannot explain prominent dyspnoea and fatigue. The prevalence of out-of-proportion pulmonary hypertension in COPD is estimated to be very close to the prevalence of idiopathic pulmonary arterial hypertension. Cor pulmonale, defined as right ventricular hypertrophy and dilatation secondary to pulmonary hypertension caused by respiratory disorders, is common. More studies are needed to define the contribution of cor pulmonale to decreased exercise capacity in COPD. These studies should include improved imaging techniques and biomarkers, such as the B-type natriuretic peptide and exercise testing protocols with gas exchange measurements. The effects of drugs used in pulmonary arterial hypertension should be tested in chronic obstructive pulmonary disease patients with severe pulmonary hypertension. In the meantime, the treatment of cor pulmonale in chronic obstructive pulmonary disease continues to rest on supplemental oxygen and a variety of measures aimed at the relief of airway obstruction.