<p>Left ventricular (LV) dysfunction is a known risk factor for adverse outcomes in patients with repaired tetralogy of Fallot (rTOF). While cardiovascular magnetic resonance (CMR) is the reference standard for LV assessment, two-dimensional echocardiography (2DE) remains the most accessible tool for surveillance. The accuracy of commonly used 2DE methods, Simpson’s biplane (SiBP) and 5/6 area-length (AL), in the setting of right ventricular (RV) dilation and altered interventricular septum configuration remains uncertain. We conducted a retrospective analysis of pediatric patients with rTOF seen between 2013 and 2023 who underwent both CMR and 2DE within a 90-day interval. Patients were excluded if an intervention occurred between studies. LV end-diastolic volume (EDV), end-systolic volume (ESV), and ejection fraction (EF) were measured using SiBP and AL methods and compared to CMR values. Agreement was compared using standard agreement metrics, with subgroup analyses stratified by RV dilation severity. 2DE reproducibility was assessed using intraclass correlation coefficient (ICC). Of 154 patients (2013–2023), 62 met inclusion criteria based on image quality. Median age at CMR was 15 years, with median indexed RVEDV of 137 mL/m². Both 2DE methods underestimated LVEDV and LVEF compared to CMR, with SiBP method performing slightly worse than AL (LVEDV percent error: 19% vs. 4%; LVEF percent error: 8% vs. 6%). Increasing RV dilation was associated with worsening percent error for LVEDV but not EF (<i>p</i> ≤ 0.04). Both techniques showed excellent reproducibility. In pediatric patients with rTOF, the AL method showed slightly better agreement with CMR than SiBP for LVEDV and was comparable for LVEF. These findings indicate that the AL method is a practical alternative for LV assessment in rTOF despite altered LV geometry. Increasing RV dilation adversely affects LV volume assessment by 2DE. This study underscores the importance of CMR for LV assessment in rTOF.</p>