The present trial investigates a possible use of oral bosentan, which is currently approved for the treatment of symptoms of pulmonary arterial hypertension (PAH) Class III and IV, to patients suffering from PAH Class II.

Inclusion Criteria:

• PAH NYHA Class II
• Significant elevation of mean pulmonary arterial pressure
• Significant elevation of pulmonary vascular resistance at rest
• Limited 6-minute walk distance

Exclusion Criteria:

• PAH secondary to portal hypertension, complex congenital heart disease or reverse shunt
• Restrictive or obstructive lung disease
• Significant vasoreactivity
• Treatments for PAH (within 4 weeks of randomization)