Although most asymptomatic patients with the Wolff-Parkinson-White syndrome have a good prognosis, some die suddenly. Electrophysiological testing may identify patients at possible risk of sudden death. The mechanism of sudden death in these patients is believed to result from ventricular fibrillation due to atrial fibrillation with rapid anterograde conduction over the accessory pathway. Consequently, we performed electrophysiological studies in 40 asymptomatic patients with the Wolff-Parkinson-White syndrome. Certain electrophysiological properties clearly identified these patients: (1) in most patients sustained reciprocating tachycardia could not be induced and this explains the absence of symptoms of regular fast palpitations; (2) the incidence of inducible sustained atrial tachyarrhythmias (30%), of short RR intervals between pre-excited beats (20%) and of risk of sudden death (12.5%) was similar to the incidence in symptomatic patients with the Wolff-Parkinson-White syndrome and reciprocating tachycardia. Because of the ease with which transoesophageal study can be performed we think that the asymptomatic Wolff-Parkinson-White syndrome should be systematically evaluated so as to reassure patients with the benign form that they can lead a normal life and take part in sport and secondly to define the real prognosis of the patients whose characteristics suggests a risk of sudden death.