Introduction: The safety and efficacy of IV procainamide is well established in adults, however data in pediatrics is more limited. Methods: Prospective cohort study including children <18 yrs old who received IV procainamide for supraventricular tachycardia (SVT) or junctional ectopic tachycardia (JET) from June 2020 to May 2021. A severity of arrhythmia score (SAS, table), including arrhythmia density score (ADS), hemodynamic severity and need for adjunctive antiarrhythmic therapy was calculated pre and post procainamide. Full success was defined as decrease of the ADS to 0 and partial success as decrease by ≥50%. Results: Fifty-three patients (6.9 [IQR 1.2-34.9] months) received IV procainamide for treatment of SVT (n = 25, 47%) or JET (n = 28, 52%), including 44 (83%) with congenital heart disease (CHD) and 39 (74%) following cardiac surgery. Fifty (93%) patients were given a bolus (median 5 mg/kg, range 2.5-13 mg/kg) and 52 (96%) received an infusion (median 20 mcg/kg/min, range 10-30 mcg/kg/min). Full success was observed in 28 (53%) patients, partial success in 13 (25%) and failure in 12 (23%); 7 (19%) patients required adjunctive IV antiarrhythmic therapy. Median SAS decreased from 6 (IQR 4-8) to 1 (IQR 1-5) following procainamide. Procainamide bolus >5 mg/kg was associated with greater decrease in SAS (median decrease 5 [4-6] vs 2 [0-4], p = 0.04). Children with CHD (median decrease 1 [IQR 0-4] vs 4 [IQR 4-5], p = 0.04), those in the early post-operative period (median decrease 2 [IQR 0-4] vs 4 [IQR 4-5], p = 0.03) and those with JET (median decrease 2 [IQR 0-4] vs 4 [IQR 2-5], p = 0.04) had less improvement in SAS score with procainamide. Conclusions: IV procainamide is effective for treatment of SVT and JET in children, and associated with decreased SAS even in patients without complete rhythm control. Higher procainamide bolus dose should be considered for increased efficacy. Future studies are needed to identify optimal doses in pediatrics.