The possibility that the Wolff-Parkinson-White syndrome may be an hereditary anomaly has been suggested in recent years by the records of several cases in the same family. This observation is further supported by this report of its occurrence in 3 generations of one family, including a set of identical and fraternal twins. Serial electrocardiographic tracings obtained on one of the 16-month-old fraternal twins document the property of retrograde conductivity postulated for an accessory atrioventricular bridge as an explanation for the mechanism of supraventricular tachycardia in the pre-excitation syndrome. The operation of a completed retrograde re-entry mechanism, involving transmission through both a normal and an anomalous atrioventricular conduction path, was also demonstrated.