Abstract Body (Do not enter title and authors here): Background: Progressive systolic dysfunction is considered an important risk factor for Fontan failure. Previously we have shown that principal strain (PS) is a novel 3D concept that provides insights into 3D deformation of single right ventricle (SRV). In this study, we plan to apply the concept of PS in patients with single left ventricle (SLV). Hypothesis: PS may predict the risk of composite outcomes in post-Fontan patient with SLV. Methods: We prospectively enrolled 48 post-Fontan patients with SLV (median age: 12.4 years), 64 with SRV, and 102 age-matched controls (12.4 years). We performed 3D speckle-tracking echocardiography (3D-STE) to measure magnitude and direction of PS (PS angle), magnitude of secondary strain (SS), other conventional strains, which were described as absolute values, and LV volumetric indices. PS depicts both magnitude and direction of principal myocardial contraction, whereas SS defines a lower intensity strain in the perpendicular direction. SLV group included tricuspid atresia (41.7%), double inlet left ventricle (37.5%), and pulmonary atresia with intact ventricular septum (20.8%). A composite outcome, experienced by 10 patients in SLV group, comprised of any of the following: Fontan failure, take-down of the Fontan, and clinically significant arrhythmia. Results: In the SLV group, the magnitudes of PS and SS were reduced in most walls, except for the lateral wall compared to controls (global PS (GPS): 30.5 ± 3.2 vs. 34.6 ± 2.8%, global SS (GSS): 12.1 ± 3.0 vs. 15.7 ± 2.9 %, p < 0.001, respectively). All PS angles in SLV group had a significantly oblique orientation, except for lateral walls and were similar to those of SRV (global PS angle: 31.4 ± 6.7 vs. 33.0 ± 6.5 vs. 27.6 ± 5.6 degrees in SLV vs. SRV vs. controls, p<0.001, Figure 1). Among the 3D-STE indices, the magnitude of PS in the anterior wall was the best predictor of composite outcomes in SLV group (AUC: 0.734, cut off: 27.1%, Figure 2). There was an increased risk of the composite outcomes, even after adjusting for age and SpO₂ (OR: 0.69 [95% CI: 0.48 – 0.98]; P < 0.04, Table 1). Conclusion: PS analysis is a newly described method in children that provides not only the magnitude but also the angles of cardiac deformation. It may offer incremental value in predicting risk of composite outcomes in post-Fontan patients.