The primary objective of this study is to assess the safety and tolerance of changing patients currently on bosentan to ambrisentan for the treatment of pulmonary arterial hypertension.

The therapy of pulmonary arterial hypertension (PAH) has been revolutionized with the development and subsequent instruction of oral endothelin receptor antagonists (ERA). The first approved ERA, bosentan (Tracleer, Actelion, Inc.) is an effective drug widely used throughout the world in the therapy of PAH. Newer ERA's, with purported advantages over the first approved drug have since been tested and subsequently been approved for the therapy of PAH in the USA and other countries including ambrisentan (Letairis, Gilead Sciences, Inc.). However, there is little data available on the efficacy, safety and tolerability of the elective change from oral bosentan to oral ambrisentan in PAH.

Inclusion Criteria:

1. Patients followed routinely in the pulmonary vascular disease clinic at the University of Alabama in Birmingham, greater than or equal to 19 years of age.
2. World Health Organization (WHO) PAH Type I
3. WHO class I-IV symptoms (no functional class exclusion).
4. On bosentan, twice a day, with a maximum daily dose of 250mg, on a stable dose for 3 months with no clinical indication to discontinue the drug (i.e., increased liver function studies or other intolerance). Patients may be on other drug therapies for PAH, and also may be on oxygen therapy (intermittent or continuous).

Exclusion Criteria:

1. Known intolerance or allergy to ambrisentan.
2. Prior therapy with ambrisentan.
3. Current therapy with two phosphodiesterase-5 inhibitors.
4. Change in other approved therapy for PAH (including phosphodiesterase-5 inhibitors and prostanoids) within 4 weeks of baseline study visit.
5. Planned addition of prostanoid for clinical reasons within 3 months of baseline study visit.
6. Active participation in another clinical study involving the medical therapy of PAH.
7. Uncontrolled systemic hypertension or angina pectoris
8. Serum creatinine greater than 2.5 at or within 4 weeks of baseline.
9. Serum liver function studies greater than 3 x normal at or within 4 weeks of baseline study visit.
10. In the opinion of the investigator, a change in PAH therapy would present significant risk to the subject.
11. In the opinion of the investigator, the participant is unlikely to survive for 12 weeks after study entry.
12. In the opinion of the investigator, the participant is likely to undergo lung or heart-lung transplantation within 12 weeks of study entry.
13. A woman of childbearing potential who is not using an acceptable form of contraception.
14. Pregnancy.
15. In the opinion of the investigator, a participant who is not capable or willing to follow the study procedures.

Subjects described in baseline are those that completed the study

There is bias in the study due to the fact that subjects have been proven to tolerate the initial drug in the study (bosentan) and adequate time was not allowed for subjects to be able to tolerate ambisentan