To evaluate the effect of three doses of oral sildenafil (20, 40 and 80 mg three times a day [TID]) on exercise capacity, as measured by the 6-Minute Walk test, as well as the safety and tolerability, after 12 weeks of treatment in subjects with pulmonary arterial hypertension who are aged 18 years and over. To investigate the plasma concentration-effect relationship and to determine the population pharmacokinetic (PK) parameters.

Inclusion Criteria:

Included patients had primary pulmonary arterial hypertension, pulmonary hypertension secondary to connective tissue disease, or pulmonary hypertension with surgical repair at least 5 years previously; a mean pulmonary artery pressure greater than or equal to 25 mmHg and a pulmonary artery wedge pressure of less than or equal to 15 mmHg at rest, via right heart catheterisation within 21 days prior to randomisation; and a baseline 6-Minute Walk test distance of 100 m to 450 m.

Exclusion Criteria:

Subjects with congenital heart disease, pulmonary hypertension due to thromboembolism, HIV, chronic obstructive airway disease, congestive heart failure or schistosomiasis; subjects with significant (ie > 2+) valvular disease other than tricuspid regurgitation or pulmonary regurgitation; and acutely decompensated heart failure within the previous 30 days; subjects with pulmonary hypertension secondary to any other etiology other than those specified in the inclusion criteria.