This is a clinical research study designed to evaluate an investigational new medication called sitaxsentan for the treatment of pulmonary arterial hypertension (patients with NYHA functional class II, III or IV). The purpose of this study is to evaluate the safety and effectiveness of two different doses of sitaxsentan, compared to placebo (inactive treatment) for the treatment of pulmonary arterial hypertension. Patients who complete this trial may be eligible to take part in an extension trial (Protocol FPH01-X). Eligible patients who receive placebo in the 12-week study cross over to receive sitaxsentan for the extension trial.