Long QT syndrome (LQTS) gene mutation carriers with indeterminate electrocardiogram frequently escape clinical diagnosis. We assessed the use of epinephrine bolus injection in revealing T-wave abnormalities.We recruited 30 genotyped asymptomatic LQTS gene carriers with nondiagnostic QT interval and 15 controls. Electrocardiogram was recorded with body surface potential mapping after an intravenous epinephrine bolus. T-wave morphology was determined as normal, biphasic, inverted, bifid, or combined pattern.Long QT syndrome carriers and healthy controls had different T-wave profiles (P = .027). Of controls, 12 (80%) of 15 had no change or biphasic appearance, whereas only 10 (33%) of 30 of LQTS carriers had so. Bifid or combined pattern occurred in 15 (50%) of 30 in LQTS and in 6 (60%) of 10 in the LQT3 subgroup but only in 1 (7%) of 15 of healthy.Modification of ventricular repolarization with low-dose epinephrine injection helps to distinguish silent LQTS mutation carriers. This concerns also the LQT3 subtype, which may escape tests.