Echocardiographic studies of interventricular septal motion were performed in 26 consecutive patients with the Wolff Parkinson White (WPW) syndrome and in ten normal subjects. All patients with types A or B preexcitation were subclassified into groups I to IV on the basis of their electrocardiogram utilizing the method of Boineau and associates. In all 14 patients with type A (Group III or IV) pre excitation, the motion of the interventricular septum and posterior left ventricular wall motion were normal. However, in 11 patients with type B (Group I) WPW an abnormal septal movement was noted. This was characterized in ten patients by an early systolic posterior motion, a subsequent anterior movement in mid systole, and the usual posterior septal motion beginning in late systole. In eight patients, including the one without early systolic posterior movement of the septum, the late systolic posterior movement was interrupted by a prominent septal notch. One patient with type B (Group II) WPW was studied and exhibited normal septal and posterior wall motion. In one patient with a spontaneous change in the QRS complex from normal to a type B (Group I) WPW pattern, the septal motion was initially normal and abruptly changed following the first WPW beat. The onset of abnormal interventricular septal motion with type B pre excitation QRS complexes strongly suggests that abnormal septal movement may be related to an altered sequence of ventricular depolarization during right ventricular pre excitation.