Idiopathic pulmonary arterial hypertension (PAH) was diagnosed in a 54-year-old man 5 years ago. He had a history of obesity and systemic hypertension. PAH was suggested by echocardiography and was confirmed by right heart catheterization (RHC), demonstrating a pulmonary artery pressure of 74/24 mm Hg (mean, 47 mm Hg), normal pulmonary capillary wedge pressure and cardiac output, and a pulmonary vascular resistance of 6.3 Wood units. There was no intracardiac shunt or significant response to vasodilator administration. Further testing revealed no pulmonary thromboembolism, rheumatologic disease, HIV infection, or interstitial lung or airway disease. There was no history of anorexigen use. Obstructive sleep apnea had been diagnosed years earlier and continued to be treated successfully with positive airway pressure. Thus, while a contribution of left ventricular diastolic dysfunction or obesity hypoventilation syndrome could not be definitively excluded, the patient appeared to have idiopathic PAH. The patient was in functional class III but declined therapy with IV epoprostenol (Flolan; Gilead; Foster City, CA). Bosentan (Tracleer; Actelion; Allschwil, Switzerland) and daytime supplemental oxygen were begun 3 and 4 years later, respectively.