Progress in treatment of pulmonary hypertension has been impaired by the lack of formal clinical trials. This is now beginning to change, and the impact on our approach to treating patients with pulmonary hypertension in substantial. As with other relatively uncommon medical disorders, randomized, controlled, multi-center trials are needed to assess the safety and efficacy of potential therapeutic modalities. Treatments showing promise at the level of small pilot studies within a single center should be studied more rigorously.