In the past decade, three classes of medications have been approved for the treatment of pulmonary arterial hypertension. A review of the clinical trial data for the prostanoids, endothelin antagonists, and phosphodiesterase-5 inhibitors has shown that all agents have similar efficacy on the 6-min walk distance over 12 to 16 weeks, which was the primary end point in the randomized clinical trials. However, little is known about their long-term efficacy or about how these drugs affect the underlying disease, if at all. Successful therapy is currently defined as an improvement in exercise tolerance over a 4-month period. Future trials need to better characterize how therapies affect the pulmonary vasculature pathologically, biologically, and hemodynamically, and whether survival is actually improved.