Nitric oxide (NO) is a free-radical gas that is an important signaling molecule in pulmonary vessels. Endogenous NO produced in endothelial cells from oxygen and L-arginine diffuses into smooth muscle cells in the vascular wall and causes vasodilatation. NO that diffuses into the blood vessel lumen is avidly bound by hemoglobin and does not cause important systemic vasodilatation. Inhaling low levels of NO rapidly and safely decreases pulmonary artery hypertension in many patients without causing systemic hypotension. In hypoxemic newborns with pulmonary hypertension, clinical studies indicate that inhaled NO increases systemic oxygen levels and decreases the requirement for extracorporeal membrane oxygenation. NO also has been observed to regulate cell proliferation. Recent studies suggest that inhaled NO selectively modulates the pulmonary artery proliferative response that is associated with lung injury. These later studies may indicate that inhaled NO can be applied to attenuate or prevent pulmonary artery disease in patients with injured lungs.