BACKGROUND: To better define determinants of mortality in patients with univentricular physiology, a database registry was created of patients born in 1985 or earlier with Fontan surgery who were followed up at Children’s Hospital Boston. METHODS AND RESULTS: A total of 261 patients, 121 of whom (46.4%) were women, had a first Fontan surgery at a median age of 7.9 years: right atrium-to-pulmonary artery connection in 135 (51.7%); right atrium to right ventricle in 25 (9.6%); and total cavopulmonary connection in 101 (38.7%). Over a median of 12.2 years, 76 (29.1%) died, 5 (1.9%) had cardiac transplantation, 5 (1.9%) had Fontan revision, and 21 (8.0%) had Fontan conversion. Perioperative mortality decreased steadily over time and accounted for 68.4% of all deaths. In early survivors, actuarial freedom from death or transplantation was 93.7%, 89.9%, 87.3%, and 82.6% at 5, 10, 15, and 20 years, respectively, with no significant difference between right atrium to pulmonary artery versus total cavopulmonary connection. Late deaths were classified as sudden in 7 patients (9.2%), thromboembolic in 6 (7.9%), heart failure-related in 5 (6.7%), sepsis in 2 (2.6%), and other in 4 (5.2%). Most sudden deaths were of presumed arrhythmic origin with no identifiable predictor. Independent risk factors for thromboembolic death were lack of antiplatelet or anticoagulant therapy (hazard ratio [HR], 91.6; P=0.0041) and clinically diagnosed intracardiac thrombus (HR, 22.7; P=0.0002). Independent predictors of heart failure death were protein-losing enteropathy (HR, 7.1; P=0.0043), single morphologically right ventricle (HR, 10.5; P=0.0429), and higher right atrial pressure (HR, 1.3 per 1 mm Hg; P=0.0016). CONCLUSIONS: In perioperative survivors of Fontan surgery, gradual attrition occurs predominantly from thromboembolic, heart failure-related, and sudden deaths.