INTRODUCTION: The Fontan circulation is prone to deterioration in functional status over time. Invasive studies have shown that Fontan patients require maximum ventricular power to maintain cardiac output at the expense of lower ventricular efficiency. OBJECTIVES: To non-invasively assess the biophysical properties of the aorta and determine ventricular-vascular interaction and efficiency in the Fontan circuit. METHODS: Twenty-five patients with a Fontan circulation and 17 Controls were assessed prospectively using echocardiography/Doppler methods. The carotid pressure waveform was recorded using applanation tonometry. RESULTS: Fontan patients were older (15.0 vs 10.4 yrs; p<0.002). Their aortic valve cross-sectional area (3.8 vs 2.0 cm², p<0.0001) was larger and peak left ventricular outflow tract flow (356 vs 244 cm³/s; p<0.003) higher, and their cardiac index (3.64 vs 4.68 L/min/m², p<0.05) lower than Controls. Pulse wave velocity (455 vs 337 cm/s), elastic pressure-strain modulus (300 vs 173 mmHg, p<0.0006) and arterial stiffness index (3.36 vs 2.10, p<0.0004) were higher in Fontan patients. Total (1019 vs 743 mW, p<0.02) and mean (878 vs 657 mW, p<0.04) ventricular hydraulic power were higher and efficiency lower (84 vs 89 %, p<0.0003) in Fontan patients. The Tei-Index (0.49 vs 0.29, p<0.001) was also higher in Fontan patients. CONCLUSIONS: The biophysical properties of the aorta and ventricular-vascular coupling in patients with a Fontan circulation are abnormal. These contribute to increased power expenditure, reduced ventricular efficiency and global function in these patients.