Prolonged QRS duration on the electrocardiogram has been found to predict adverse arrhythmic events in patients late after repair of tetralogy of Fallot. Whether QRS duration can also predict inducible ventricular tachycardia (VT) at electrophysiologic study is unknown. Between 1984 and 1995 we studied 135 survivors of tetralogy of Fallot surgery whose age at surgery was 34 days to 37 years (3.7 ± 3.9, median 2.5) and age at electrophysiologic study was 1.4 to 43 years (9.7 ± 8.2, median 6.7). QRS duration was 80 to 240 ms (137 ± 29) and ≤180 ms in 9 patients. Sustained VT was induced in 22 patients (monomorphic in 17). Induced sustained monomorphic VT was related to QRS duration, right ventricular dimension, H-V interval, and presence of symptoms. QRS duration was also related to induced sustained monomorphic VT by multivariate analysis. QRS duration ≤ 180 ms was 35% sensitive and 97% specific for induced sustained monomorphic VT. QRS duration was related to induced sustained monomorphic VT even when only asymptomatic patients were analyzed. A QRS duration ≤180 ms was 100% sensitive and 96% specific for detecting clinical VT. Prolonged QRS duration on the electrocardiogram is associated with induced sustained monomorphic VT on electrophysiologic study. The finding of prolonged QRS duration should suggest the need for further testing to determine the risk of adverse arrhythmic events in patients after repair of tetralogy of Fallot, even if they are asymptomatic.