As a result of the intraoperative trauma sinus node dysfunction and AV conduction defects have been observed. In addition bifascicular block (RBBB with LAH) may occur after repair of a ventricular septal defect. This electrocardiographic pattern indicates a high risk of developing a trifascicular block only in those patients with reversible AV block in the early postoperative period. After closure of an atrial septal defect, atrial fibrillation can develop as well as ventricular tachycardia and fibrillation after repair of a ventricular septal defect. The latter seems to be the underlying mechanism of sudden cardiac death occurring late after operation, especially in patients with Fallot tetralogy. In addition to the scar, persistent structural myocardial changes seem to be an important factor. Electrophysiological investigation is indicated in all symptomatic patients especially for detection and treatment of ventricular tachyarrhythmias. However, there is yet no proof that sudden death can be prevented by antiarrhythmic treatment of all ventricular premature beats in the endangered patients.