• Treatment-emergent adverse events [ Time Frame: From first inhalation of study drug to day before end of 12-week treatment period ] [ Designated as safety issue: Yes ]
• Treatment-emergent serious adverse events [ Time Frame: From first inhalation of study drug to the day before the end of 12-week treatment period ] [ Designated as safety issue: Yes ]
• Adverse events leading to premature discontinuation of study drug [ Time Frame: From the first inhalation of study drug to discontinuation ] [ Designated as safety issue: Yes ]

Patients with symptomatic idiopathic (IPAH) or familial (FPAH) pulmonary arterial hypertension in NYHA class II to IV currently being treated with a stable dose of either bosentan or sildenafil and who complete PROWESS 15 will be enrolled in the PROWESS 15 Extension study. This is a double-blind (12 week), randomized study to compare the safety and tolerability of inhaled iloprost power disc-15 and power disc-6 in patients with symptomatic PAH. After completion of the double blind period, patients will be entered in the open label period using iloprost power disc-15.

• Experimental: iloprost power 15
• Active Comparator: iloprost power 6

Inclusion Criteria:

1. Signed informed consent prior to initiation of any study mandated procedure,
2. Patients with symptomatic idiopathic or familial pulmonary arterial hypertension in NYHA functional class II to IV who have completed study AC-063A301,
3. Women of childbearing potential must have a negative urine pregnancy test and must use an adequate method of contraception during the study and for 28 days after discontinuation of the study drug.

Exclusion Criteria:

1. Pulmonary arterial hypertension related to any condition other than those specified in the inclusion criteria,
2. Pulmonary arterial hypertension associated with significant venous or capillary involvement (PCWP > 15 mmHg), known pulmonary veno-occlusive disease, or pulmonary capillary hemangiomatosis,
3. Moderate to severe obstructive lung disease: forced expiratory volume in 1 second/forced vital capacity (FEV1/FVC) < 70% and FEV1 < 65% of predicted value after bronchodilator administration,
4. Moderate to severe restrictive lung disease: total lung capacity (TLC) < 60% of predicted value,
5. Pregnant or breast-feeding women,
6. Systemic hypertension (systolic blood pressure > 160 mmHg or diastolic blood pressure > 100 mmHg on repeated measurement),
7. Systolic blood pressure < 95 mmHg,
8. Moderate to severe hepatic impairment, i.e., Child-Pugh Class B or C,
9. Chronic renal insufficiency defined by serum creatinine > 2.5 mg/dL (221 μmol/L) or ongoing dialysis,
10. Clinically relevant bleeding disorder or active bleeding,
11. Known hypersensitivity to iloprost or any of its excipients.