We present a 14-year-old pre-transplant boy with elevated pulmonary pressure and plastic bronchitis after Fontan operation despite stent fenestration, who improved with long-term oral administration of the endothelin antagonist, bosentan. Bosentan improved the patient's symptoms and aortic saturation, World Health Organization class, maximal and sub-maximal exercise capacity, Borg dyspnea index, invasively measured mean pulmonary pressure, pulmonary blood flow and pulmonary vascular resistance. This is the first literature report examining the effect of endothelin antagonism in failing Fontan circulation and showing clinical, exercise and hemodynamic improvement with bosentan in a pre-transplant Fontan patient with high pulmonary pressure and plastic bronchitis.