A 13-month-old boy with intermittent preexcitation (Lown-Ganong-Levine type) and paroxysmal tachycardias of 280 to 300 beats per minute died suddenly with cardiac arrest. At necropsy the only abnormalities were in the heart. There were multifocal Purkinje cell tumors in the conduction system, including one directly at the bifurcation of the His bundle. In addition, a fault in the central fibrous body was filled with an accessory communication between the mid-portion of the atrioventricular node and both the interventricular septum and the His bundle. This accessory communication connected with the Purkinje cell tumor. Although an atrioventricular nodal bypass was thus present and could account for the preexcitation and for reentrant tachycardias, there is also the anatomic basis for an automatic arrhythmia originating within one or more of the Purkinje cell tumors, particularly the one directly within the His bundle. Dispersion of elements of the conduction system within the central fibrous body resembled the normal fetal pattern. Whether this persistence of fetal dispersion of the atrioventricular node and His bundle within the central fibrous body is causally related to the presence of multifocal Purkinje cell tumors or is only coincidence merits further consideration.