Pulmonary arterial hypertension is a progressive disease that has a high rate of mortality. For these reasons, early treatment is essential. Treatment choices for pulmonary arterial hypertension are limited by drug tolerability, drug cost and inconvenience associated with administration techniques and dosing schedules. Therefore, a therapy that provides oral dosing with limited side effects would prove useful in managing many patients. Sildenafil citrate, the first and highly publicized oral medication to receive approval from the U.S. Food and Drug Administration for erectile dysfunction, has recently been approved for treatment of pulmonary arterial hypertension. This review summarizes the normal physiology of the pulmonary vasculature, and the pathophysiology involved in pulmonary arterial hypertension and the role of sildenafil in its treatment.