Our goal is to determine clinically in Pulmonary Arterial Hypertension patients if associations exist between the efficacy and toxicity of sitaxsentan, bosentan, and ambrisentan and several gene polymorphisms in several key disease-specific and therapy specific genes. Also characterized is the relationship between these polymorphisms and the severity of Pulmonary Arterial Hypertension using either baseline hemodynamic or clinical surrogates for disease severity.