BAY63-2521 Riociguat leads to the relaxation of smooth muscle cells in pulmonary arteria and may also inhibit abnormal remodeling of lung blood vessels. In patients with pulmonary arterial hypertension Riociguat showed to reduce the pulmonary blood pressure and improved the right heart function without unacceptable side effects. Here dose of Riociguat will be adjusted over 8 weeks then a Maintenance Phase of 16 weeks follows. Patients with Pulmonary Arterial Hypertension treated with stable doses of Phosphodiesterase Type-5 Inhibitors (Eg Sildenafil, Tadalafil) not appropriately responding to therapy will be included. Based on previous evidence and on the different modes of action an improvement of exercise capacity, heart function and quality of life may be expected if PDE5i treatment is transitioned to riociguat.