Introduction: Repolarization heterogeneity is associated with increased risk of arrhythmia and sudden death, which is well known in patients with long QT syndrome (LQTS). In patients with repaired congenital heart disease (CHD), repolarization property can be variable between myocardium and it can increase myocardial electrical instability, but the link is not identified yet. Here we report 6 patients with repaired CHD and LQTS. Results: Two patients with tetralogy of Fallot had KCNH 2 and KCNQ1 mutation respectively. A patient with tricuspid atresia and notched T wave in 5 precordial leads was confirmed as SCN5A mutation a year after Fontan operation. Two patients with septal defect had KCNH2 mutation. A patient with ventricular septal defect repair and pacemaker implantation due to complete postoperative atrioventricular block had ventricular fibrillation at her age of 23 years and implantable cardiac defibrillator was implanted. On her genetic analysis, she had polymorphisms in SCN5A and KCNH2 gene. Median QTc of these patients was 0.499 seconds (range 0.457-0.574) at diagnosis and median QRS duration was 97 ms (range 70-144). Beta-blocker was prescribed in all patients. Age at genetic diagnosis for long QT syndrome was median 7.5 years (range 1.5-22). Conclusion: Electricomechanical instability of repaired CHD can make repolarization heterogeneity worse in patients with LQTS and the risk for ventricular arrhythmia and sudden cardiac death can much increase. Therefore, patients with CHD need close follow-up for QT intervals and we need further study for the treatment and risk stratification for the patients with CHD and LQTS.