The Ventavis® (iloprost) Registry is a multicenter, observational, U.S.-based study that longitudinally follows patients with pulmonary arterial hypertension (PAH) who have been receiving therapy with Ventavis® for at least 3 months. Patients diagnosed with WHO Group I PAH who are on a stable regimen of commercial Ventavis® will be followed for a maximum of 2 years from the time of enrollment. Data will be collected via patient interview and review of the medical record. Quarterly data collection will include capture of medications and Ventavis® adherence data.