Atrial flutter/fibrillation (AFL/AF) is a late complication in adults with repaired tetralogy of Fallot (TOF). Its effects on long-term prognosis are not fully understood. We evaluate the impact of AFL/AF in adults with repaired TOF on global mortality and unplanned hospitalizations during follow-up, and the predictors for AFL/AF occurrence. The presence of AFL/FA was analysed in all exams performed during the last 10 years of outpatients follow up in a unicentric cohort of repaired TOF between 1980 and 2003. Two-hundred and six patients were included; at a mean follow-up of 21 ± 8.2 years, there were 5 deaths (19.2%) in the AFL/AF group and 2 (1.1%) in those without arrhythmia (p < 0.001). Patients with AFL/AF where older at the time of the surgical repair (p < 0.001) and had a higher rate of reinterventions (p = 0.003). No differences were observed between the groups regarding the use of a transannular patch, ventriculotomy and previous palliative shunt. QRS duration was longer in patients with AFL/AF (174 ± 33.4) when compared to those without arrhythmia (147 ± 39.6; p < 0.0001). Age at surgery, QRS duration, and tricuspid regurgitation ≥ moderate were independent risk predictors for AFL/AF. In the multivariate analysis, atrial flutter/fibrillation and QRS duration were predictors of death and hospitalization. AFL/AF is associated with an increased risk of death and hospitalization during the follow-up of patients with repaired TOF. Early detection of AFL/AF and their predictors is an essential step in the evaluation of such population.