Pulmonary arterial hypertension (PAH) is a progressive disorder with a poor prognosis. It is characterized by sustained elevation of pulmonary artery pressure (PAP) and pulmonary vascular resistance (PVR). It is defined hemodynamically by a mean PAP over 25 mm Hg, a pulmonary arterial wedge pressure of 15 mm Hg or less (which excludes left sided lesions), and a PVR of 3 or more Wood units (240 dyn.sec.cm-5). Patients are limited by exertional dyspnea, pre- or true syncope, chest pain, and edema/ascites when right heart failure supervenes. PAH afflicts predominantly young women and the diagnosis is often delayed. Three processes contribute to progressive arterial narrowing: vasoconstriction, vascular remodeling, and thrombosis in situ. The diagnosis of PAH must be confirmed and its etiology must be identified before appropriate therapy can be instituted. Right heart catheterization is necessary to establish the diagnosis, severity, and prognosis of PAH and to ascertain its etiology and to evaluate vasoreactivity, which guides therapy. Treatment of PAH includes vasodilators, supplemental O2, anticoagulation, diuretics, digoxin, intravenous inotropic therapy for decompensated right ventricular failure, and lung or combined heart-lung transplantation for those patients who continue to deteriorate with a poor quality of life despite pharmacologic therapy. Calcium channel blockers are beneficial in a small minority of patients. Prospective, controlled, randomized trials of approved vasodilator agents have enrolled a large proportion of women (70-85%). Agents such as the endothelin-1 receptor antagonist bosentan, the phosphodiesterase-5 inhibitor sildenafil, and the prostanoids have been shown to improve symptoms, exercise capacity, and, in most instances, delay clinical worsening. The clinical outcomes of patients with PAH have improved with the judicious use of contemporary therapies.