Inclusion Criteria:

   - Patients must be a permanent resident of the United States

   - Documentation of the following hemodynamic parameters by right heart catheterization

      - Mean pulmonary arterial pressure (PAPm) ≥ 25 mm Hg at rest and,

      - Pulmonary artery wedge pressure (PAWP) ≤ 15 mm Hg (or > 15 mmHg if justified)

   - Radiologic confirmation that chronic thromboembolic disease is the cause of the
   pulmonary hypertension by

      - One or more mismatched perfusion defect(s) by lung ventilation/perfusion scan,
      and

      - Confirmation of chronic thromboembolic disease by evidence of bands/webs, vessel
      narrowing or occlusion seen on CT pulmonary angiogram (CTA), conventional
      angiography or MR angiography (MRA).

   - Patients must be diagnosed with CTEPH within 6 months of being considered for study
   eligibility (signing of consent to participate). The date of diagnosis will be defined
   as when both hemodynamic criteria have been met and chronic thromboembolic disease is
   confirmed to be the cause of the pulmonary hypertension by an abnormal V/Q scan and
   the presence of chronic thromboembolic disease on CTA, MRA or pulmonary angiography.
   Hemodynamic and radiologic criteria can be met at separate time points; the most
   recently met criteria time point will be defined as the date of diagnosis.

   - Prior to enrollment all subjects must have the diagnosis of CTEPH confirmed by the
   Adjudication Committee.

Exclusion Criteria:

   - Patients unwilling or unable to provide written consent for participation in the
   study. Appropriate surrogate consent will be obtained for pediatric patients as
   defined by each investigational site's IRB.

   - Patients with an underlying medical disorder with an anticipated life expectancy less
   than 2 years.

   - Patients who do not meet inclusion criteria including:

      - Have not had documentation of hemodynamic criteria by right heart catheterization
      as outlined in the inclusion criteria

      - Do not have radiologic confirmation of chronic thromboembolic disease as outlined
      in the inclusion criteria

      - Meet the criteria for inclusion into WHO Groups I, II, III, or V