Ventricular fibrillation (VF) is a well-known but rare complication of the Wolff-Parkinson-White syndrome (WPW). Clinical and electrophysiological data of 23 patients with spontaneous VF were compared with data from 100 consecutive patients with WPW without VF but with symptomatic supraventricular tachycardia. The 23 patients were collected in a multicentre retrospective study in seven European centres. VF occurred in only one patient who was receiving antiarrhythmic drugs, and was the first manifestation of the syndrome in six. No significant differences were found between those with VF and without VF in age, complaints of palpitations, syncope, and presence of structural heart disease. The retrograde effective refractory period of the accessory pathway, the atrial refractory period and the fastest atrial pacing rate with 1:1 anterograde conduction over the accessory pathway were similar in both groups. Significant differences were found for sex, permanent pre-excitation on the electrocardiogram, type of documented supraventricular tachyarrhythmias, shortest RR interval less than or equal to 220 ms during spontaneous atrial fibrillation (AF), inducibility of supraventricular tachycardias, ventricular effective refractory period less than or equal to 190 ms, mean shortest RR interval during induced AF less than or equal to 180 ms and presence of multiple accessory pathways.(ABSTRACT TRUNCATED AT 250 WORDS)