Pulmonary arterial hypertension (PAH) refers to a group of diseases characterized by high pulmonary artery pressure of unknown mechanism. Primary pulmonary hypertension (PPH) is the idiopathic subset of PAH that affects a mostly young population and is more common in females than in males. A familial form of PPH accounts for about 6% of cases, and its autosomal dominant gene was recently identified. Pulmonary arterial hypertension is histologically characterized by endothelial and smooth muscle cell proliferation, medial hypertrophy, and thrombosis in situ. The pathogenesis of PAH remains unclear. Elevated pulmonary vascular resistance seems to result from an imbalance between locally produced vasodilators and vasoconstrictors, in addition to vascular wall remodeling. Nitric oxide, a locally produced selective pulmonary vasodilator, appears to play a central role in the pathobiology of PAH.