Objective: This study assessed clinical condition at midterm follow-up after total cavopulmonary connection for a functionally univentricular heart performed on children younger than 5 years. Methods: Thirty-four Fontan patients (median age 10.4 years, range 6.8-20.7 years, 22 boys, median follow-up 7.8 years, 5.0-17.8 years) underwent electrocardiography, Holter monitoring, bicycle exercise testing, cardiac magnetic resonance imaging, and N-terminal prohormone brain natriuretic peptide (NT-pro-BNP) analysis. Results: Twenty-three patients (68%) were in sinus rhythm. Holter monitoring demonstrated normal mean heart rate, low maximal heart rate, and no clinically significant arrhythmias or sinus node dysfunction. With maximal bicycle ergometry (n = 19), maximum workload (60% of normal), maximum heart rate (90% of normal), and maximal oxygen uptake (69% of normal) were all significantly lower in the Fontan group than in a control group (P < .001). Variables of submaximal exercise indicated less efficient oxygen uptake during exercise in all Fontan patients. Ejection fraction was lower than in control subjects (59% ± 13% vs 69% ± 5%, P < .001). Mean end-diastolic and end-systolic volumes and ventricular mass were higher than in control subjects (P < .001). Mean NT-pro-BNP levels were increased relative to reference values, but only 8 patients had levels above the upper reference limit. Conclusion: At midterm follow-up, Fontan patients were in acceptable clinical condition, with preserved global ventricular function, moderately decreased exercise capacity, and NT-pro-BNP levels within reference range. Systemic ventricular mass was elevated, however, suggesting contractility-afterload mismatch. Long-term consequences for ventricular function merit further investigation.