Patients must have been diagnosed with WHO Group 1 Pulmonary Arterial Hypertension: Idiopathic, Familial or Associated with (APAH) Collagen vascular disease, congenital systemic-to-pulmonary shunts, portal hypertension, Drugs and toxins , other (thyroid disorders, glycogen storage disease, Gaucher disease, hereditary hemorrhagic telangiectasia, hemoglobinopathies, myeloproliferative disorders, splenectomy) Associated with significant venous or capillary involvement, Pulmonary veno-occlusive disease, Pulmonary-capillary hemangiomatosis. Patients currently therapy must include sitaxsentan, bosentan, or ambrisentan OR patients must have previously been treated with sitaxsentan, bosentan or ambrisentan for 4 months or longer.