Case-history of a four-year-old boy with Ebstein's anomaly of the tricuspid valve with WPW syndrome. Severe tricuspid insufficiency and repeatedly occurring tachydysrhythmias soon led to cardiac decompensation and death before the intended cardiac operation. Non-invasive clinical, ECG, X-ray and ECHO examinations correlated with catheterization, electrophysiological and pathological findings. In addition to the dislocated, malformed tricuspid valve the septal cusp of which was fused with the ventricular septum the authors found another additional atrioventricular muscular connection between the lower part of the septum of the right atrium and right ventricle.