Pulmonary arterial hypertension (PAH) is defined as a sustained elevation of pulmonary arterial pressure greater than 25 mm Hg at rest or greater than 30 mm Hg with exercise, with a mean pulmonary capillary wedge pressure and left-ventricular end-diastolic pressure of less than 15 mm Hg. Idiopathic PAH (IPAH) includes cases formerly referred to as primary pulmonary hypertension. Other categories of PAH include PAH associated with collagen vascular disease or human immunodeficiency virus, congenital heart disease with left-toright intracardiac shunts, portal hypertension, and persistent pulmonary hypertension of the newborn. Regardless of the cause, the histopathology is similar in PAH.