The prequisite of better therapeutic results in patients with Ebstein's anomaly of the tricuspid valve is more perfect diagnosis not only of the severity of the malformed valve but also of other associated defects incl. the anomalous accessory bundle of Kent which causes the development of dangerous tachydysrhythmias. The authors present their own experience with the localization of the bundle of Kent by mapping the endocardium of the atria in seven children before a cardiac operation. In all ventricular preexcitation type B with the bundle of Kent in the posterior lower portion of the atrial septum was found. In two non-operated patients who died with tachydysrhythmia which could not be controlled, the bundle of Kent was confirmed by histological examination, in three it was severed by operation, two patients are waiting to be operated. The authors recommend interruption of the bundle of Kent always concurrently with operation of the malformed tricuspid valve and other associated cardiac defects.