This is a randomized, double-blind study evaluating the effectiveness of BSF 208075 in treating patients with moderate to severe pulmonary hypertension. A four-week Screening Period will be followed by 12 weeks of Treatment. After a subject qualifies for the study, the subject will be randomized to one of four doses of BSF 208075 (1.0, 2.5, 5.0 or 10.0 mg po qd). Subjects randomized to the 1.0 or 2.5 mg dose groups will receive their respective doses of BSF 208075 each day throughout the 12-week Treatment Period. Subjects in the two other dose groups will begin treatment at 2.5 mg per day for two weeks and then their dose will be increased to 5.0 mg for an additional two weeks. After two weeks of treatment at 5.0 mg, subjects randomized to the 10.0 mg dose group will undergo a final up-titration. After reaching the randomized dose level, subjects will receive their assigned dose throughout the Treatment Period. Subjects will remain on the randomized treatment through Week 12. In the event that a subject is not tolerating study drug, dose adjustment is permitted during the Treatment Period. Upon completion of the 12-week Treatment Period subjects will either complete a four-week Down-titration Period or enter an optional 12-week Open-label Extension. All subjects that choose to participate in the Open-label Extension will be unblinded and have their dose of BSF 208075 optimized based on the subjects response during the Treatment Period.