This study will investigate the effects of the combination of bosentan and sildenafil. Patients with symptomatic PAH treated with a stable dose of sildenafil equal to or greater tha 20 mg t.i.d. for at least 12 weeks will be randomized to placebo or bosentan 125 mg b.i.d. All randomized patients will be treated with study drug until the predefined target number of morbidity/mortality events is reached.