In our study, we will compare the exercise capacity, echocardiographic measures of cardiac function, and the overall quality of life in patients with the Fontan before and after a six-week period of sildenafil administration. As a control, the same group of patients will take a placebo for a six-week period, also with before and after testing. We hypothesize that oral sildenafil will result in significant improvements in exercise capacity, energy levels, and echocardiographic measures of cardiac function and output in our study participants. We are hopeful that the findings of this investigation will directly help children and young adults with Fontan physiology.