• Change From Baseline to Week 24 in 6 Minute Walk Distance (6MWD) [ Time Frame: Baseline to Week 24 ] [ Designated as safety issue: No ]
  

• Percent of Participants With no Clinical Worsening of Pulmonary Hypertension at Week 24. [ Time Frame: 24 weeks ] [ Designated as safety issue: No ]
  Clinical worsening: occurrence of death, lung transplantation, hospitalization for PH, atrial septostomy, a change to chronic prostanoid or sildenafil treatment due to protocol-defined worsening criteria, or study withdrawal due to the addition of other clinically approved PH therapeutic agents
  
  
• Change From Baseline to Week 24 in WHO Functional Class [ Time Frame: Baseline to Week 24 ] [ Designated as safety issue: No ]
  Change from baseline in WHO functional class (WHO) at Week 24 is expressed as the incidence of patients that improved, had no change or worsened. WHO categories range from 1 to 4 with the worse category at 4. Improvement = a category change from baseline of at least -1: change of -3 (e.g.: WHO from 4 to 1), change of -2 (e.g.: WHO from 3 to 1), change of -1 (e.g.: WHO from 2 to 1). Inversely, patients worsening are those with a category change from baseline of at least +1. No change in WHO functional class represents the pecentage of patients with a change in category from baseline of 0.
  
  
• Percent Change From Baseline to Week 24 in B-type Natriuretic Peptide (BNP) [ Time Frame: Baseline to Week 24 ] [ Designated as safety issue: No ]
  
• Long-term Survival [ Time Frame: Week 24 ] [ Designated as safety issue: No ]
  Defined as not dying during study participation
  
  
• Change From Baseline to Week 24 in Borg Dyspnea Index [ Time Frame: Baseline to Week 24 ] [ Designated as safety issue: No ]
  Change from Baseline to Week 24 in Borg Dyspnea Index. The Borg Dyspnea Index of Perceived Exertion Scores range from 0 to 10. Best and Worst values are: 0 (Best) to 10 (Worst). Scales are described as rating of breathlessness and its description: 0= none; 0.5= very,very slight (just noticeable); 1= very slight; 2=slight; 3= moderate; 4= somewhat severe; 5= severe; 7= very severe; 9= very, very severe; and 10= maximum.
  
  
• Change From Baseline to Week 24 in SF-36 Health Survey Physical Functioning Scale [ Time Frame: Baseline to Week 24 ] [ Designated as safety issue: No ]
  Change from baseline to Week 24 in the SF-36 health survey physical functioning scale. 10 activities are rated by health limitations using 3 categories (1= Yes, limited a lot; 2= Yes, limited a little; and 3= No, not limited at all). The best score is 3 and the worst score is 1. Scores are transformed by substracting the unit by the lowest raw score and dividing by the raw score range. The scores are then standardized with the 1998 General US population mean and standard deviation. Finally, the scores are transformed to the norm-based scoring with a mean of 50 and standard deviation of 10.
  
  
• Monotherapy Treatment Status [ Time Frame: Week 24 ] [ Designated as safety issue: No ]
  Defined by: no addition of sildenafil, iloprost, treprostinil, or epoprostenol to ongoing ambrisentan treatment
  
  
• Failure-free Treatment Status [ Time Frame: Week 24 ] [ Designated as safety issue: No ]
  Defined by: occurrence of death, lung transplantation, or study withdrawal due to the addition of other clinically approved PAH therapeutic agents
  
  

This study will enroll up to 200 subjects with PAH and in PH subgroups including PH associated with interstitial lung disease (ILD); PH due to chronic thromboembolic disease or sickle cell disease; PH associated with chronic obstructive pulmonary disease (COPD); PAH associated with congenital heart defects; and PAH associated with HIV. Subjects may be receiving prostacyclin or sildenafil therapy at baseline, and subjects who previously discontinued either bosentan, sitaxsentan, or both, due to liver function test abnormalities are eligible.