Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) is a genetic arrhythmia syndrome characterized by frequent ventricular ectopy and polymorphic, classically bidirectional ventricular tachycardia with physical or emotional stress, which also carries a risk of ventricular fibrillation and sudden death, despite no structural heart abnormality. Treatment consists of beta-blockers and/or calcium channel blockers, but up to 30% of patients require implantable cardioverter-defibrillators (ICDs) due to recurrent symptoms on medical therapy. In an animal model, flecainide was found to directly target the molecular defect in CPVT. In a retrospective clinical study in patients with CPVT we have seen improvement of ventricular ectopy on exercise tests when flecainide is added to standard therapy. We propose a prospective trial of flecainide added to standard therapy in CPVT patients to test the hypothesis that flecainide will reduce ventricular ectopy on exercise testing compared to placebo plus standard therapy.