Dofetilide may induce aberrant intraventricular conduction due to its Class III effect. This report describes an atrial fibrillation patient in whom intraventricular conduction was studied before and after dofetilide using multiple endocardial recordings. Dofetilide provoked aberrant conduction during atrial fibrillation, and aberrancy could be mimicked with programmed atrial stimulation after restoration of sinus rhythm. However, during right ventricular stimulation, isolated bundle branch reentrant beats were recorded after induction of critical retrograde conduction delays. This occurred in the setting of relatively large differences in refractoriness between the right bundle branch and the right ventricular myocardium. This favored distal retrograde bundle branch block during ventricular extrastimulation, in turn enhancing bundle branch reentry. This potential proarrhythmic mechanism deserves close attention in the further development of dofetilide and also of other new "pure" Class III agents.