Background: Ventricular arrhythmia and sudden cardiac death late after repair of tetralogy of Failer are devastating complications in adult survivors of early surgery, but their prediction remains difficult. Methods: We examined surgical, electrocardiographic, and late haemodynamic data, and their relation to clinical arrhythmia and sudden death occurring over 10 years, in a multicentre cohort of patients with repaired tetralogy, who were alive in 1985. Results: Of 793 patients (mean age at repair 8·2 years [SD 8], mean time from repair 21·1 years [8·7]) who entered the study, 33 patients developed sustained monomorphic ventricular tachycardia, 16 died suddenly, and 29 had new-onset sustained atrial flutter or fibrillation. Electrocardiographic markers (QRS duration, QRS rate of change between 1985 and 1995) were significantly greater in the ventricular tachycardia and sudden-death groups. Older age at repair was associated with a higher risk of sudden death and atrial tachyarrhythmia. Pulmonary regurgitation was the main underlying haemodynamic lesion for patients with ventricular tachycardia and sudden death, whereas tricuspid regurgitation was for those with atrial flutter/fibrillation. Despite adverse haemodynamics, no patient who died suddenly had undergone late reoperation. Conclusion: Arrhythmia and sudden death are important late sequelae for patients after repair of tetralogy of Fallot. The electrophysiological and haemodynamic substrate of sudden death resembled that of sustained ventricular tachycardia, with pulmonary regurgitation being the predominant haemodynamic lesion. Preservation or restoration of pulmonary valve function may thus reduce the risk of sudden death. Furthermore, electrocardiographic markers can help to identify patients at risk.