The purpose of this study is to determine the safety and efficacy of Iloprost in subjects that have Pulmonary Arterial Hypertension who are concurrently taking bosentan (Tracleer TM).

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.

Inclusion Criteria:

• Diagnosis of PAH due to PPH, connective tissue disease, HIV, or repaired (≥ 1 year) ASD, VSD, or PDA
• NYHA Functional Class III or IV
• On bosentan for at least 16 weeks, with the dose stable (maximum dose 125 mg BID) for at least 8 weeks
• Age 12-75 years, of either gender
• Six minute walk distance 100-425 meters at Baseline

Exclusion Criteria:

• Any new long-term treatment for PAH added within the last 4 weeks
• Any therapy with a PDE (phosphodiesterase), L -arginine or a prostaglandin, concurrently, or within the last 4 weeks
• PAH related to chronic thromboembolic disease, portopulmonary disease, or any etiology other than PPH, connective tissue disease, HIV, or repaired ASD, VSD, or PDA