Pulmonary hypertension is the major cardiovascular complication of chronic respiratory disorders and its development is a sign of poor prognosis. Pulmonary circulation is characterized by its low vascular tone and the response to hypoxia by vasoconstriction. Pulmonary endothelium plays an important role in modulating these characteristics. Structural abnormalities of pulmonary arteries in pulmonary hypertension affect preferentially the intimal layer and may damage the endothelial cells. Endothelial dysfunction of pulmonary arteries has been recognized in the different forms of pulmonary hypertension. Vasodilators are recommended for the treatment of primary pulmonary hypertension. However, in pulmonary hypertension associated with chronic respiratory disorders, both systemic and selective pulmonary vasodilators are not indicated since they may worsen gas exchange due to the inhibition of hypoxic vasoconstriction. The most effective treatment of pulmonary hypertension associated with chronic hypoxic lung diseases is long-term oxygen therapy.