Purpose This study proposes to investigate the safety, tolerability, and feasibility of transitioning patients with PAH from intravenous or subcutaneous prostacyclin analogs to inhaled treprostinil using a defined protocol. Twenty-one patients from three PH specialty referral centers will be enrolled in this 12-week, prospective, open-label study. If subjects meet inclusion and exclusion criteria, they will be switched from intravenous or subcutaneous prostacyclin analogs to inhaled treprostinil according to a defined transition protocol.