Inclusion Criteria:

• Have a current diagnosis of symptomatic PAH classified by one of the following:

  1. primary pulmonary hypertension (PPH) - also known as idiopathic pulmonary arterial hypertension (IPAH);
  2. PAH associated with connective tissue diseases;

  3. PAH associated with one of the following congenital heart defects:

     1. repaired ASD, VSD or PDA greater than one year post-operative
     2. un-repaired secundum ASD (with resting oxygen saturation greater than 88 percent in room air measured by oximeter)
• World Health Organization (WHO) functional class II, III, IV
• Greater than 12 and less than 75 years of age
• Women of childbearing potential must be using two forms of medically acceptable contraception (at least one barrier method)

• Have a cardiac catheterization within 6 months before study entry that shows the following values:

  1. mean pulmonary artery pressure (PAPm) greater than 25 mmHg (at rest),
  2. pulmonary capillary wedge pressure (PCWP) or left ventricular-end diastolic pressure less than 15 mmHg, and
  3. pulmonary vascular resistance (PVR) greater than 3 mmHg/L/min.

Exclusion Criteria:

• Portal hypertension or chronic liver disease
• ALT or AST levels greater than 1.5 times the upper limit of normal at the Screening Visit
• Contraindication to treatment with an endothelin receptor antagonist
• Recent history of abusing alcohol or illicit drugs
• Chronic renal insufficiency
• Pregnant or breastfeeding
• Atrial septostomy within 30 days before study entry
• Previous failure on bosentan because of safety concerns of the lack of clinical response