A Extension Study of Udenafil in Adolescents

Inclusion Criteria: 1. Males and females with Fontan physiology who participated in the FUEL trial or, if they did not participate in FUEL, those who are 12 to less than 19 years of age at enrollment. 2. Participant consent or parental/guardian consent and participant assent. 3. Participant fluent in English, Spanish, or Korean. 4. Current anti-platelet or anticoagulant therapy. Exclusion Criteria: 1. Height \< 132 cm. 2. Weight \< 40 kg. 3. Hospitalization for acute decompensated heart failure within the last 12 months. 4. Current intravenous inotropic drugs. 5. Undergoing evaluation for heart transplantation or listed for transplantation. 6. Diagnosis of active protein losing enteropathy or plastic bronchitis within the last three years, or a history of liver cirrhosis. 7. Known Fontan baffle obstruction, branch pulmonary artery stenosis, or pulmonary vein stenosis resulting in a mean gradient of \> 4 mm Hg between the regions proximal and distal to the obstruction as measured by either catheterization or echocardiography. 8. Single lung physiology. 9. Maximal VO2 less than 50% of predicted for age and gender at enrollment. 10. Severe ventricular dysfunction assessed qualitatively by clinical echocardiography within six months prior to enrollment. 11. Severe valvar regurgitation, ventricular outflow obstruction, or aortic arch obstruction assessed by clinical echocardiography within six months prior to enrollment. 12. Significant renal, hepatic, gastrointestinal or biliary disorders that could impair absorption, metabolism or excretion of orally administered medications. 13. Inability to complete exercise testing at baseline screening. 14. History of PDE-5 inhibitor use (with the exception of FUEL participation) within 3 months before study onset. 15. Use of any other drug to treat pulmonary hypertension within 3 months before study onset. 16. Known intolerance to oral udenafil. 17. Frequent use of medications or other substances that inhibit or induce CYP3A4. 18. Current use of alpha-blockers or nitrates. 19. Ongoing or planned participation in another research protocol that would either prevent successful completion of planned study testing or invalidate its results. 20. Noncardiac medical, psychiatric, and/or social disorder that would prevent successful completion of planned study testing or would invalidate its results. 21. Cardiac care, ongoing or planned, at a non-study center that would impede study completion. 22. For females: Pregnancy at the time of screening, pregnancy planned before study completion, or refusal to use an acceptable method of contraception for study duration. 23. Unable to abstain or limit intake of grapefruit juice during the duration of the trial. 24. Refusal to provide written informed consent/assent. 25. In the opinion of the primary care physician, the subject is likely to be non-compliant with the study protocol. 26. History of clinically significant thromboembolic event, as adjudicated by study Investigators.