Pulmonary arterial hypertension (PAH) is a term that was coined in 1998 at the World Health Organization Symposium on Pulmonary Hypertension at Evian, France. Because a clinically useful classification for pulmonary hypertension was lacking, the late Alfred Fishman proposed a clinical classification system for pulmonary hypertension which has been widely adopted. Its purpose was to provide a guide to physicians who encounter a patient with pulmonary hypertension of uncertain etiology. The classification system helped direct the clinical evaluation of the patient so that underlying disease(s) that might be causing or contributing to the pulmonary hypertension could be identified. PAH (also known as Category 1) includes patients who have an elevation in pulmonary artery pressure with a normal pulmonary wedge pressure, either alone or in association with many other diseases.