Study to Assess Safety and Efficacy of Ambrisentan in Subjects With Pulmonary Arterial Hypertension.

Inclusion Criteria: * 18 years of age or order * Current diagnosis of either PPH orPAH secondary to the scleroderma spectrum of disease, systemic lupus erythematosus, anorexigen use, or HIV infection at the Screening Visit * Right heart catheterization, completed prior to Screening Visit must meet pre-specified hemodynamic criteria * Female subjects of childbearing potential must have a negative serum pregnancy test and must agree to use a reliable double barrier method of contraception until study completion and for at least four weeks following their final study visit * Male subjects must be informed of the potential risks of testicular tubular atrophy and infertility associated with taking this study drug and queried regarding his understanding of the potential risks as described in the Informed Consent Form Exclusion Criteria: * PAH due to or associated with congenital heart disease, coronary artery disease, left heart disease, interstitial lung disease, chronic obstructive pulmonary disease, veno-occlusive disease, chronic thrombotic and/or embolic disease, or sleep apnea * Portopulmonary hypertension * Bosentan within four weeks prior to Screening * Phosphodiesterase type V inhibitor or chronic prostanoid therapy within four weeks prior to Screening * IV inotrope use within two weeks prior to Screening * ALT or AST lab value that is greater than 1.5 times the upper limit of normal * Pulmonary function tests not meeting pre-specified criteria * Contraindication to treatment with an ERA * History of malignancies other than basal cell carcinoma of the skin or in situ carcinoma of the cervix within the past five years * Females who are pregnant or breastfeeding