In patients with the Fontan circulation, systemic venous return flows passively towards the lungs. Because of the absence of the subpulmonary ventricle, favourable blood flow patterns with minimal energy loss are clinically relevant. The region where the inferior vena cava, the hepatic veins and the extracardiac conduit join (IVC conduit junction) is a potential source of increased energy loss. The aim of this study was to evaluate the relationship between geometry and blood flow patterns in the IVC conduit junction with associated kinetic energy and energy loss using 4-dimensional flow magnetic resonance imaging (MRI). METHODS: Fourteen extracardiac conduit-Fontan patients underwent 4-dimensional flow MRI. The IVC conduit junctions were ranked into 3 groups for 3 categories: the geometry, the flow complexity and the conduit mean velocity. The relative increase in the mean velocity from the IVC to the conduit (representing IVC conduit mismatch) was determined. The peak kinetic energy and mean kinetic energy and energy loss were determined and normalized for volume. In 4 of 14 patients, adverse geometries led to helical flow patterns and/or acute changes in flow direction. For each category, the most adverse IVC conduit junctions were associated with an approximate 2.3 3.2-fold and 2.0 2.9-fold increase in kinetic energy and energy loss, respectively. The IVC conduit mismatch is strongly correlated with the mean kinetic energy and energy loss (r = 0.80, P = 0.001 and q = 0.83, P 0.001, respectively) and with body surface area in patients with 16-mm conduits (r = 0.88, P = 0.010). The IVC conduit junction is a potential source of increased energy loss. Junctions with increased energy loss showed: (i) a distorted geometry leading to adverse blood flow patterns and/or (ii) the IVC conduit mismatch. Sixteen-millimetre conduits appear to be inadequate for older patients.