Assess liver stiffness using ultrasound point shear wave elastography (US P-SWE) in children before and after the Fontan operation. Eighteen children undergoing the Fontan operation were prospectively enrolled. Eight US P-SWE measurements were obtained from the right hepatic lobe before surgery, and at multiple postoperative time points. Temporally related inferior vena cava pressure (IVC) data was collected from medical records, when available. Changes in mean liver shear wave speed (SWS) were assessed using a mixed-effect model with post hoc Tukey correction. Changes in IVC pressure were evaluated using the Wilcoxon signed-rank test. A p value less than 0.05 was considered significant. Mean age at enrolment was 33.5 +/- 10.5 months. Baseline mean liver SWS was normal at 1.18 +/- 0.29 m/s, increased to 2.28 +/- 0.31 m/s at 2.5 +/- 1.2 days (p < 0.0001) and to 2.22 +/- 0.38 m/s at 7.5 +/- 1.4 days (p < 0.0001). Five subjects returned at a mean of 185 +/- 28 days, and mean liver SWS remained elevated at 2.08 +/- 0.24 m/s (p < 0.0001). Mean IVC pressure increased from 7.2 +/- 2.6 mmHg at baseline to 16.44 +/- 3.3 mmHg at 2.2 +/- 0.8 days post-surgery (p = 0.004). The Fontan operation immediately and chronically increases liver stiffness and IVC pressure. Our study provides further evidence that congestion is a key driver of Fontan-associated liver disease. aEuro cent The Fontan operation triggers immediate hepatic congestion and marked liver stiffening. aEuro cent Congestion, not fibrosis, drives early increased liver stiffness in Fontan patients. aEuro cent Hepatic congestion persists chronically for months after the Fontan operation. aEuro cent Congestion confounds shear wave elastography as a post-Fontan liver fibrosis biomarker.