Abstract The class IC antiarrhythmic drug, propafenone, has been used successfully for the prevention of atrial fibrillation (AF), with few reported adverse cardiovascular effects. 1–3 Propafenone depresses the rate of rise and amplitude of phase 0 of the action potential, slowing conduction in the atrium. 4 Although it has mild β-adrenergic antagonist effects, it has relatively less effect on atrioventricular refractoriness, although in AF the ventricular response may slow. 5 With slower atrial arrhythmias such as atrial flutter, where less concealment of conduction occurs, it is possible that the same effect on ventricular rate may not be seen. This study defines the incidence of atrial flutter developing in a heterogeneous population of patients with recurrent AF and observes the ventricular response during this arrhythmia.