Objective: Effects of timing of inception of amiodarone antiarrhythmic treatment after corrective surgery for congenital heart defects are not yet well defined. Methods: In a 10-year prospective observational study, 71 of 2651 patients (2885 procedures, 2106 bypass procedures) received amiodarone for newly detected postoperative atrial (n = 70) or ventricular (n = 7) tachyarrhythmia: early treatment (initiation within 60 minutes of arrhythmia detection) and late treatment (after that). From an interim analysis, a treatment protocol was established (32 patients). Inotropic requirements were monitored, as were heart rate, blood pressure, central venous pressure, and sedation requirements. Results: With early treatment (n = 29), delay before start of amiodarone was 40.7 ± 21.9 minutes, versus 227.7 ± 228.1 minutes (P < .001) with late treatment (n = 42). There were significant benefits of early treatment for times to rate (155.9 ± 299.8 vs 407.6 ± 376.9 minutes, (P < .001) and rhythm control (400.4 ± 845 vs 1038.5 ± 1158.4 minutes, P < .001), reduction in dose needed for rate control (28.2 ± 45.2 vs 66.5 ± 137.5 mg, P < .025), and significant reduction in pediatric cardiac intensive care unit stay (3.32 ± 1.9 vs 5.26 ± 4.27 days, P < .01). There were continuous improvements in heart rate, blood pressure, and filling pressures without additional inotropic requirements or side effects. Conclusion: Early treatment of postoperative tachyarrhythmia with amiodarone according to a standardized treatment protocol is safe and has beneficial effects on arrhythmia control and pediatric cardiac intensive care unit stay.