Patients with Wolff-Parkinson-White (WPW) pattern of ventricular pre-excitation may develop paroxysmal re-entrant tachyarrhythmias through the Kent bundle and, less commonly, atrial fibrillation. WPW patients are at risk of sudden death when a rapid ventricular response occurs during atrial fibrillation due to conduction through the accessory pathway. Conduction properties of the accessory pathway and atrial vulnerability, which is the propensity to develop atrial fibrillation, are important parameters for evaluation in these patients. The former can be assessed by means of noninvasive tests, such as stress and pharmacological tests, and with electrophysiological study; the latter only by electrophysiological study. There is no indication for treatment of asymptomatic patients. Antiarrhythmic prophylaxis is required in patients with previous episodes of atrial fibrillation with rapid ventricular response, in patients with paroxysmal re-entrant tachycardias and rapid conduction through the accessory pathway, and in patients with frequent episodes of re-entrant tachycardias of long duration. Vaughan-Williams class IC anti-arrhythmic drugs (propafenone, flecainide) are the first choice for drugs in patients with rapid anterograde conduction through the accessory pathway due to their high efficacy and low incidence of adverse effects, while beta-blockers (atenolol, nadolol) are indicated for patients with re-entrant tachycardias and low conduction capacity through the bypass tract. When pharmacological therapy is ineffective, surgical or catheter ablation of the accessory pathway may be considered.