RATIONALE: Tetralogy of Fallot (TOF) is a right-sided dominant cardiac lesion with a stenotic right ventricular (RV) outflow tract and hypertrophied RV, which could result in septal shift and RV dilation. This study evaluated the RV/left ventricular (LV) diameter ratio at end-systole in association with clinical outcomes in children who underwent repair of TOF. The main aim of this study was to determine the association of RV/LV diameter ratio at end-systole with the outcomes of TOF repair. The specific objectives were to determine whether the preoperative (demographic and echocardiographic data) parameters were associated with the clinical outcomes of TOF repair and to determine whether the intraoperative (cardiopulmonary bypass time and aortic cross-clamp time) parameters were associated with the clinical outcomes of TOF repair. METHODOLOGY: This prospective cohort single-center study enrolled 112 TOF patients aged 1 to 18 years admitted for surgical repair. The RV/LV ratio was measured using echocardiography preoperatively, indexed to patient’s weight and body surface area, and associated with its postoperative cardiac complications using logistic regression analysis. RESULTS: Cumulative incidence of cardiopulmonary complications was at 28.7% (32 of 112). Reperfusion lung injury had the most cases (11.6%), followed by low cardiac output syndrome (9.8%) and postpericardiotomy syndrome (5.4%). Both RV and LV diameters, indexed to patient’s weight and body surface area, (P = 0.0051, P < 0.001), RV/LV ratio (P < 0.001), and LV end diastolic size with z scores (P < 0.001), showed statistically significant results. Increase in RV/LV ratio is associated with adverse postoperative clinical outcomes in pediatric patients who underwent repair of TOF (P < 0.001). Significant preoperative parameter showed that stunting is associated with complications postoperatively (P = 0.017), whereas for the intraoperative parameters, prolonged cardiopulmonary bypass time and aortic cross-clamp time were also associated with postoperative complications. CONCLUSION: The increased value of RV/LV end-systolic diameter ratio at 1.63 (±0.256), which incorporates both pathologic septal shift and RV dilation, is associated with higher incidence of postoperative cardiac complications among patients with tetralogy of Fallot.