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 and who are on a stable regimen of commercial Ventavis® will be followed for a maximum of 2 years from the time of enrollment. All data will be collected via patient interview and/or review of the medical record, as well as from I-neb® downloads using INSIGHT software. Adherence with Ventavis® will be compared before and after targeted instruction on inhalation techniques by a respiratory therapist or designee, as well as before and after participation in PAH patient support groups (if applicable). Quarterly data collection will include capture of PAH and non-PAH medications and Ventavis® adherence data; Ventavis® adherence data will also be collected one month after study entry.