A mild degree of pulmonary hypertension is commonly observed in patients with acute respiratory distress syndrome (ARDS). Vasoconstriction plays a dominant role in increasing the mean closing pressure of the pulmonary vascular bed. Pulmonary hypertension increases right ventricular afterload, but right ventricular failure rarely occurs in patients with ARDS. Although the initial magnitude of pulmonary hypertension is not an indicator of mortality, pulmonary artery pressure increases in nonsurvivors but not in survivors when followed for 7 days. Pulmonary vascular tone is closely related to gas exchange, and the increase in pulmonary artery pressure is a protective mechanism to minimize ventilation-perfusion mismatching in ARDS. Consequently, generalized pulmonary vasodilation by intravenous vasodilators aggravates arterial hypoxaemia. Conversely, selective administration of vasodilators, such as inhaled nitric oxide, in well-ventilated lung units improves gas exchange by diverting pulmonary blood flow to better oxygenated regions. However, preliminary results from large prospective randomized controlled trials indicate that inhaled nitric oxide does not reduce mortality nor the duration of mechanical ventilation in patients with ARDS.