Morphological demonstration by serial sections of accessory atrio-ventricular connections in patients with Wolff-Parkinson-White syndrome is time-consuming. However, knowledge of the extent and relative position of the accessory muscle bridge is the essential basis for surgical therapy. In three patients with electrophysiologically and electrocardiographically demonstrated WPW syndrome the accessory pathway is demonstrated at the expected site. In three controls there was no evidence of such atrio-ventricular connections.