Wolff-Parkinson-White (WPW) syndrome is characterized by a wide spectrum of clinical conditions: many subjects remain symptom free during the whole life, others suffer from paroxysmal episodes of reciprocating atrioventricular tachycardia, a minority of patients present with episodes of atrial fibrillation. The latter is the most dreadful arrhythmia because in the presence of a short refractory period of the Kent bundle it can produce very high ventricular rates, degenerate into ventricular fibrillation and cause sudden death. Sudden death however is very rare in the WPW syndrome. In 1988 the European registry on "sudden death in the WPW syndrome" collected 26 cases of various centres. This study demonstrated that sudden death occurs in 73% of cases in symptomatic subjects while in the remaining 27% it is an unexpected event in previously asymptomatic subjects. While sudden death is very rare, high frequency atrial fibrillation producing hemodynamic deterioration is not so rare, although its precise incidence is unknown. In hospitalized WPW patients it is reported in 10-30% of cases. One of the main problems in symptomatic and asymptomatic patients with the WPW pattern is to identify the subjects at risk of high frequency arrhythmias. Many Authors suggested that endocavitary or transesophageal electrophysiologic study can be useful in this regard if its results are evaluated by a multiparametric approach. In our opinion the main parameters to be evaluated are: presence or absence of retrograde conduction of the Kent bundle (which is necessary for the initiation of atrioventricular reciprocating tachycardia which is the commonest trigger of atrial fibrillation); atrial vulnerability; shortest and mean RR intervals during induced atrial fibrillation.(ABSTRACT TRUNCATED AT 250 WORDS)