Eligible trial volunteers must meet the following criteria:

-PAH diagnosed with right heart catheterization
-May have idiopathic PAH or may have PAH after repair of a congenital heart defect
-World Health Organization (WHO) functional class II or III
-Be between three months and 12 years four months old
-Pulse oximetry of greater than or equal to 88 percent on room air at rest
-Bosentan-naive patients or patients already treated with bosentan (as monotherapy) and/or with: calcium channel blocker, intravenous (IV) or inhaled prostanoid, or oral sildenafil
-In bosentan-naive patients, baseline PAH therapy has to be stable for at least three months prior to screening