Sildenafil is an oral medication that has been used to treat patients with pulmonary hypertension, a disease in which there is abnormally elevated pressure in the vessels of the lung. In this disease, the resistance in the lungs is abnormally high, severely limiting the ability of the heart to keep up with the demands of the body. Sildenafil lowers the resistance in the vessels of the lungs and has been shown to improve exercise performance in patients with this disease. We believe that Sildenafil may have a similar benefit for our patients after Fontan operation in whom cardiac output is also limited by resistance of the blood vessels in the lungs.