This study was to enroll up to 200 participants with PH due to the following etiologies: 1) PAH including idiopathic and familial PAH and PAH associated with collagen vascular disease, congenital systemic-to-pulmonary shunts (including Eisenmenger's syndrome), human immunodeficiency virus (HIV) infection, drugs and toxins, thyroid disorders, glycogen storage disease, Gaucher disease, hemoglobinopathies, and splenectomy (WHO Group 1); 2) PH associated with lung diseases and/or hypoxemia, including chronic obstructive pulmonary disease (COPD), interstitial lung disease (ILD), sleep-disordered breathing, and alveolar hypoventilation disorders (WHO Group 3); 3) PH due to proximal or distal chronic thromboembolic obstruction (WHO Group 4); and 4) PH due to sarcoidosis (WHO Group 5). Participants with left heart disease or left heart failure were excluded (WHO Group 2). Participants could be receiving prostacyclin or sildenafil therapy at baseline, and participants who previously discontinued either bosentan, sitaxsentan, or both, due to liver function test abnormalities were eligible.