Interventricular septal motion was studied by echocardiogram in 20 consecutive patients with documented Wolff-Parkinson-White (WPW) syndrome before and during electrophysiologic evaluation using His bundle recordings and pacing techniques. Characteristic abnormal interventricular septal motion was seen in 8 of 11 patients with type B WPW syndrome (groups I and II). All eight patients had electrocardiographic patterns consistent with an anomalous pathway located in the anterior right ventricular wall (group I). In five of these eight patients normalization of the QRS complex for one or more beats was accomplished and produced normalization of the septal motion in four; whereas in the fifth patient, who had an underlying atrial septal defect, the abnormal septal motion remained abnormal. All nine patients with type A WPW syndrome (groups III to V) had normal septal motion both during total preexcitation and during normalization of the QRS complex. The normalization of the abnormal interventricular septal motion with normalization of the QRS complex in type B WPW syndrome strongly suggests that the abnormal motion is related to an abnormal sequence of ventricular depolarization during preexcitation. Furthermore, persistent abnormal septal motion after normalization of the QRS complex suggests that other factors such as right ventricular volume overload may be responsible. Likewise, when abnormal septal motion occurs in the presence of type A WPW syndrome, an explanation other than preexcitation must be sought.