This study will make use of a large population of well defined patients with Pulmonary Arterial Hypertension who were enrolled in Encysive Pharmaceutical's STRIDE clinical trials or who have received bosentan or ambrisentan for 4 months or longer. This international study constitutes the largest clinical study of this deadly disease and in such has great potential to alter the clinical practice by revealing novel gene-drug interactions. This study tests the hypothesis by executing the following aims: