OBJECTIVES: The aim was to investigate at what age electrocardiographic characteristics of long QT syndrome type 3(LQT3) and Brugada syndrome(BS), based on a single SCN5A mutation, appear. BACKGROUND: The QT interval(QT) in LQT3 is prolonged during bradycardia. It is not clear yet if this is obvious in young children with a relative fast heart rate(HR). METHODS: Thirty-six children with an SCN5A gene mutation(1795insD) and 46 non-carrier siblings were investigated. In different age groups, HR, QT, QTc, and ST segment elevation on a 12-lead electrocardiogram(ECG), and HR, QT, QTc, and ΔQT after the longest pause in a Holter(recording) were evaluated. RESULTS: In all age groups, HR at rest tended to be lower in carriers than in non-carriers, and QT was longer in carriers than in non-carriers. The Brugada phenotype was found >5 years. Gender specific differences were not identified. The QT at lower HR and ΔQT were longer in carriers than in non-carriers. A QTc of< 0.44 s at the lowest HR(sensitivity 100%; specificity 88.4%) and ΔQT< 60 ms(sensitivity 100%; specificity 82.6%) were good predictors for having LQT3. CONCLUSIONS: We conclude that electrocardiographic characteristics of LQT3 and BS show age-dependent penetrance. A QT prolongation and conduction disease were present from birth onwards,whereas ST-segment elevation only developed >5 years. Good tools for clinical diagnosis of LQT3 in this family are QTc at the lowest HR and ΔQT after a pause in a Holter, even at very young age. OBJECTIVES: The aim was to investigate at what age electrocardiographic characteristics of long QT syndrome type 3 (LQT3) and Brugada syndrome (BS), based on a single SCN5A mutation, appear. BACKGROUND: The QT interval (QT) in LQT3 is prolonged during It is not clear yet if this is obvious in young children with a relative fast heart rate (HR). METHODS: Thirty-six children with an SCN5A gene mutation (1795insD) and 46 non-carrier siblings were investigated. groups, HR, QT, QTc, and ST segment elevation on a 12-lead electrocardiogram (ECG), and HR, QT, QTc, and ΔQT after the longest pause in a Holter , HR at rest tended to be lower than carriers in non-carriers, and QT was longer in carriers than in non-carriers. The Brugada phenotype was found> 5 years. Gender specific differences were not identified. The QT at lower HR and AQT were longer in carriers than in non-carriers. A QTc of <0.44 s at the lowest H CONCLUSIONS: We conclude that electrocardiographic characteristics of LQT3 and BS show age-dependent penetrance. A QT (sensitivity 100%; specificity 88.4%) and ΔQT <60 ms (sensitivity 100%; specificity 82.6% prolongation and conduction disease were present from birth onwards, whereas ST-segment elevation only developed> 5 years. Good tools for clinical diagnosis of LQT3 in this family are QTc at the lowest HR and ΔQT after a pause in a Holter, even at very young age.