Patients with preexcitation are at risk for sudden cardiac death. The pathogenesis is a rapid antegrade conduction of atrial fibrillation over the accessory pathway to the ventricle resulting in ventricular fibrillation. Today it is possible to eliminate the conduction over the accessory pathway by catheter intervention (radiofrequency ablation) with a low rate of complications and a high rate of success. In clinical practice it is therefore important to estimate the risk for sudden cardiac death in an individual patient to give an advice to the patient and the parents about the further evaluation and therapeutic strategy. The velocity of the conduction over the accessory pathway can be estimated by analysing the ECG during sinus tachycardia. If the preexcitation disappears at a relatively low heart rate, the risk for sudden death is less than in patients with persisting preexcitation at the maximal heart rate. Compared to the gold standard i.e. measurement of the refractory period of the accessory pathway during invasive electrophysiological study (EPS), the measurements at the stress ECG have been reported to be a relatively poor indicator for an elevated risk which may be explained by a high intraindividual variability of this test. This study is designed to define the clinical relevance of the stress ECG in paediatric patients with preexcitation (compared to the invasive electrophysiological study).