Estimated to occur in 0.1% to 0.3% of the population, Wolff-Parkinson-White syndrome (WPW) is a condition where atrial impulses bypass the atrioventricular node and activate the ventricular myocardium directly via an accessory pathway. Clinical clues to the diagnosis include a young patient with previous episodes of palpitations, rapid heart rate, or syncope. Although several different rhythm presentations are possible, atrial fibrillation is a not infrequent dysrhythmia seen in the WPW patient. Electrocardiographic features suggestive of WPW atrial fibrillation include irregularity of the rhythm; a very rapid ventricular response; presence of a delta wave; and a wide, bizarre QRS complex. Stable patients suspected of having this condition should not receive agents that predominantly block atrioventricular conduction, but they may be treated with procainamide or ibutilide. If instability is present, electrical cardioversion is required.