In 13 patients with Wolff-Parkinson-White syndrome (WPW) the findings from electrophysiologic studies have been correlated with the patients' histories. Four patients had had syncopes with tachycardia and at least one episode of cardiac resuscitation (group 1). Five patients with tachycardias had never had syncope (group 2). Group 1 and group 2 had anterograde conduction over an accessory atrio-ventricular pathway (AAVP) during investigation, while 4 patients had concealed pathways (group 3). Age, localization of AAVP, duration of tachycardia history and heart rate during regular tachycardia did not differ in the three groups. Atrial and anterograde effective refractory period of AAVP was shorter in group 1 (280 +/- 10 msec) than in group 2 (328 +/- 15 msec). Shortest RR-interval during atrial fibrillation was 230 +/- 30 msec in group 1 versus 295 +/- 10 msec in group 2. It is concluded that potentially dangerous tachyarrhythmias are due to short refractory period of AAVV. Patients with syncope and WPW-syndrome should undergo electrophysiologic investigation and be selected for controlled antiarrhythmic treatment or surgical dissection of AAVP.