Recent discoveries in the disease pathophysiology of pulmonary arterial hypertension have been translated into effective therapies tested in clinical trials. The studies have focused on surrogate and intermediate end points, thought to reflect quantity and quality of life, respectively. The authors present the necessary requirements for establishing the reliability and validity of such end points before they may be used dependably. The authors also review the available data, strengths, and weaknesses of potential end points in pulmonary arterial hypertension.