We determined the prevalence of sustained atrial tachyarrhythmia (AT) in adults late after repair of tetralogy of Fallot (ToF) and examined its impact on subsequent heart failure, reoperation, and mortality. Ventricular arrhythmias are associated with increased morbidity and mortality in patients with repair of ToF. The clinical impact of AT in this population has not been established. A retrospective cohort study of 242 patients with repaired ToF identified 29 patients (prevalence of 12%) with sustained episodes of AT. Patients with repaired ToF but without sustained arrhythmia (n = 213) constituted a comparison group. Baseline characteristics and clinical outcomes in the 2 groups were compared. An echocardiographic analysis compared 15 patients with AT and 15 matched for age at operation and timing of echocardiography. The development of AT was associated with substantial morbidity including congestive heart failure, reoperation, subsequent ventricular tachycardia, stroke, and death (combined events, 20 of 29 patients [69%]). The rate of combined events (congestive heart failure, stroke, and deaths) in the 213 "arrhythmia-free" patients was 30% (64 of 213 patients). Event-free survival after repair was 18 +/- 2 years for the AT group and 28 +/- 1 years for the arrhythmia-free group (p < 0.001). Patients with AT were older at surgical repair (25 +/- 16 vs 10 +/- 9 years, p = 0.001), and at most recent assessment were aged 48 +/- 12 vs 32 +/- 10 years (p = 0.001). The AT group had a higher mean right atrial volume and proportion of significant pulmonary regurgitation than matched controls. The development of AT in the adult late after ToF repair identifies patients at risk and is associated with older age at repair, a higher frequency of hemodynamic abnormalities, and increased morbidity.