Pulmonary hypertension associated with disorders of the respiratory system is defined by a pulmonary artery mean pressure above 20 mmHg at rest in stable disease. The most frequent form of precapillary pulmonary hypertension is associated with chronic obstructive pulmonary disease, because of its high prevalence. Pulmonary vascular remodeling occurs in the small pulmonary arteries and is due mainly to chronic alveolar hypoxia. Pulmonary hypertension associated with disorders of the respiratory system is usually mild to moderate, with resting pulmonary artery mean pressure ranging between 20 and 35 mm Hg. It may increase markedly during sleep, exercise or exacerbation of respiratory failure, however. Abrupt postload elevation can lead to right heart failure, an indisputable indicator of prognosis. Because the symptoms of pulmonary hypertension are minimal relative to those of the chronic hypoxic lung disease, noninvasive diagnosis is difficult, particularly in patients with chronic obstructive pulmonary disease. Oxygen therapy (at least 16 h/day) is currently the best treatment for this type of pulmonary arterial hypertension.