OBJECTIVES: To determine the feasibility of 4D flow MRI for visualization and quantification of the portal venous haemodynamics in children and young adults. METHODS: 4D flow was performed in 28 paediatric patients (median age, 8.5 years; interquartile range, 5.2-16.5), 15 with non-operated native portal system and 13 with surgically created portal shunt. Image quality assessment for 3D flow visualization and flow pattern analyses was performed. Regional 4D flow peak velocity and net flow were compared with 2D-cine phase contrast MRI (2D-PC MR) in the post-surgical patients. RESULTS: Mean 3D flow visualization quality score was excellent (mean +/- SD, 4.2 +/- 0.9) with good inter-rater agreement (kappa,0.67). Image quality in children aged >10 years was better than children =10 years (p < 0.05). Flow pattern was defined for portal, superior mesenteric, splenic veins and splenic artery in all patients. 4D flow and 2D-PC MR peak velocity and net flow were similar with good correlation (peak velocity: 4D flow 22.2 +/- 9.1 cm/s and 2D-PC MR 25.2 +/- 11.2 cm/s, p = 0.46; r = 0.92, p < 0.0001; net flow: 4D flow 9.5 +/- 7.4 ml/s and 2D-PC MR 10.1 +/- 7.3 ml/s, p = 0.65; r = 0.81, p = 0.0007). CONCLUSIONS: 4D flow MRI is feasible and holds promise for the comprehensive 3D visualization and quantification of portal venous flow dynamics in children and young adults. KEY POINTS: * 4D flow MRI is feasible in children and young adults. * 4D flow MRI has the ability to non-invasively characterize portal haemodynamics. * Image quality of 4D flow MRI is better is older children. * 4D flow MRI can accurately quantify portal flow compared to 2D-cine PC MRI.