This study will examine the effects of add-on sildenafil to bosentan monotherapy in patients with Pulmonary Arterial Hypertension. Patients on bosentan monotherapy will be followed every 6 months to assess if they have met the pre-defined treatment goals. If a patient fails to achieve these treatment goals or fails to maintain them, sildenafil will be added to their existing bosentan monotherapy. Patients will be assessed 6 months after start of combination therapy for changes in 6MWT, Borg dyspnea scale, WHO functional class, quality of life.