Vasoconstrictive signaling via endothelin receptors is not limited to primary pulmonary arterial hypertension, but has also been documented in secondary pulmonary hypertension due to congestive heart failure, including cardiac valve disease. The investigators aim to examine the clinical and physiologic effects of bosentan therapy in patients with secondary pulmonary hypertension due to severe, inoperable cardiac valve disease, using a single-center, prospective, open-label, non-randomized study of oral bosentan in outpatients with severe mitral stenosis due to childhood rheumatoid fever. Primary end-point will be exercise capacity at six months determined by six-minute walking distance and cardiopulmonary exercise testing. Secondary end-points will be symptomatic relief, echocardiographic left ventricular function and pulmonary pressure, serum pro-brain natriuretic peptide, and adverse events at six months.

Inclusion Criteria:

• Outpatient
• > 60 Yrs Old
• < 85 Yrs Old
• Stable disease
• Congestive heart failure NYHA IIIB/V
• Inoperable mitral stenosis due to childhood rheumatoid fever
• Mean pulmonary artery pressure > 40 cm H2O

Exclusion Criteria:

• Prior treatment with endothelin receptor antagonist(s)
• Hospitalization (exacerbation)
• Cardiac valve surgery