Prostacyclin improves symptoms, exercise tolerance, and survival in patients with pulmonary arterial hypertension. However, the difficulty of administration (whether intravenous, subcutaneous, or by inhalation) often causes side effects that can reduce the patient's quality of life and which may sometimes be serious. Bosentan, an orally active endothelin receptor antagonist, improves functional class and exercise tolerance in these patients. We describe the successful transition from prostacyclin to bosentan in five patients with severe pulmonary arterial hypertension who suffered serious side effects with prostacyclin treatment.