We report the case of a young man who presented with a rapid, narrow-complex atrial fibrillation. A few hours after being administered intravenous metoprolol and diltiazem for rate control, he developed intermittent ventricular preexcitation on the electrocardiogram (ECG) and experienced ventricular fibrillation, from which he was successfully defibrillated. A subsequent electrocardiogram in sinus rhythm demonstrated previously unknown Wolff-Parkinson-White pattern. A left lateral accessory pathway was successfully ablated. Wolff-Parkinson-White syndrome should be included in the differential diagnosis when a young patient presents with atrial fibrillation, even if the ventricular complexes on the ECG are not preexcited.