Dronedarone is a relatively new antiarrhythmic drug and is held to be less proarrhythmic than comparable compounds, although its proarrhythmia potential in humans has not been sufficiently evaluated. We describe a so far unreported dronedarone effect, namely a significant alteration of both the morphology and the duration of the QRS complex on the electro cardiogram in a 41-year old patient with symptomatic paroxysmal atrial fibrillation.