• Incidence and severity of adverse events associated with long-term exposure of ambrisentan in patients with PAH who have completed the Phase II AMB-220 study.
  

• Change from baseline in exercise capacity as measured by the 6-minute walk test distance; Borg Dyspnea Index; WHO Functional Classification, Subject Global Assessment; clinical worsening of PAH; failure-free treatment status
  
• and long-term survival.
  

Inclusion Criteria:

• Must have completed Visit 14/Week 24 of the AMB-220 study.
• Women of childbearing potential must have a negative urine pregnancy test at the Screening/Enrollment Visit and agree to use a reliable double barrier method of contraception until study completion and for at least four weeks following their final study visit.
• Must have completed the Down-titration Period of AMB-220 prior to enrollment in AMB-220-E and will meet the following additional criteria:
• Subjects with a diagnosis of HIV must have stable disease status at the time of Screening/Enrollment.
• Must be stable on conventional therapy for PAH for at least four weeks prior to the Screening Visit.

Exclusion Criteria:

• Chronic prostanoid therapy, or other investigational prostacyclin derivative within four weeks prior to the Screening Visit.
• IV inotrope use within two weeks prior to the Screening Visit.
• Females who are pregnant or breastfeeding.
• Contraindication to treatment with an ERA (endothelin receptor antagonist).