Wide complex tachycardias may represent a rare proarrhythmic effect of some antiarrhythmic agents. The authors describe a patient who developed a wide complex tachycardia developing during stress testing while on therapy with flecainide, and the difficulty in interpreting the electrocardiogram. This article reviews potential proarrhythmic effects of flecainide in the treatment of atrial dysrhythmias and the difficulty in differentiating associated wide complex tachycardias.