This is a randomized, placebo-controlled study in subjects with Pulmonary Arterial Hypertension (PAH) with an NYHA Class of III or IV who are receiving conventional therapy and bosentan. Subjects who fulfill the inclusion and exclusion criteria will be randomized to study drug (active or placebo) at a frequency of 6-9 inhalations per day for 12 weeks and will continue conventional therapy and bosentan. At the end of the double-blind phase study, open label Iloprost will be provided.
