Class IC antiarrhythmic drugs, specifically encainide and flecainide, have been used with increasing frequency in children and young adults with both atrial and ventricular tachyarrhythmias. As a result of the recent findings from the Cardiac Arrhythmia Suppression Trial (CAST) study, their use in adults as well as children has been questioned. Because of their potential and often dangerous side effects, the child receiving a class IC drug must be monitored very closely. We report a case of flecainide toxicity in a 2 1/2-week-old neonate who was being treated for incessant supraventricular tachycardia.