This 6-month open label study will evaluate the long term safety of bosentan (via oxygen saturation) and efficacy (exercise capacity) in patients who have completed the BREATHE-5 study (PAH related to Eisenmenger physiology). Treatment duration is 6 months.

This is a multicenter, open-label, single -arm study with bosentan, initial dose of 62.5 mg b.i.d., with a target dose of 125 mg b.i.d. All patients will be assessed for eligibility during the baseline visit (same as Week 16, end of study BREATHE-5 visit), and will have the option to enter into the open-label extension study at a starting dose of 62.5 mg b.i.d., for safety reasons. Visit 2 is required as a safety visit to assess oxygen saturation (SpO2) after 1 week of bosentan treatment. Patients will return for Visit 4 after 4 weeks of treatment, and the dose will be up-titrated to 125 mg b.i.d. or maintained at 62.5 mg b.i.d. at the judgement of the investigator. Data will be collected for a total of 24 weeks or until the sponsor decides to stop the study. If the results from Study AC-052-403 or AC-052-405 demonstrate significant safety issues, this extension study may be terminated at the sponsor's request.

Inclusion Criteria:

1. Patients stable with PAH upon completion of the BREATHE-5 randomized, double-blind, placebo- controlled 16-week study.
2. For female patients, only non-pregnant women who are surgically sterile, postmenopausal or have documented infertility (over 50 years of age and amenorrheic for at least 1 year), or those of childbearing potential using one of the following methods of contraception: Barrier-type devices (e.g., condom, diaphragm) used ONLY in combination with a spermicide. A double-barrier method is recommended; intrauterine devices (IUDs); oral or implanted contraceptives, if used in combination with a barrier method.
3. Patients providing written informed consent.

Exclusion Criteria:

1. Patients who withdrew prematurely from BREATHE-5, AC-052-405.
2. Patients who are pregnant or nursing.
3. Patients who are receiving epoprostenol or prostacyclin analogues, or are expected to receive any of these drugs during the study.
4. Patients with AST and/or ALT values greater than 3 times the upper limit of normal.
5. Patients with hemoglobin or hematocrit that is more than 30% below the normal range (patients with secondary polycythemia are permitted in the study).
6. Patients with systolic blood pressure < 85 mm Hg.
7. Patients taking phosphodiesterase inhibitors, other endothelin receptor antagonists or are receiving another investigational product.
8. Patients active on organ transplant list.
9. Patients who are receiving or expected to receive glyburide, cyclosporin A or tacrolimus.
10. Patients not able to comply with the protocol or adhere to therapy.