Molecular genetic studies in congenital long QT syndrome have characterized genes and mechanisms of arrhythmias. At least six genes encoding cardiac potassium and sodium ionic channels have been described with several mutations in each gene. The altered function produces abnormal cardiac repolarization seen on ECG as prolongation of the QT-interval and T-wave abnormalities. This may increase the propensity for ventricular arrhythmias such as Torsade de Pointe, the cause of unexpected syncope and sudden death in young patients. Clinical manifestations vary depending on the genotype present. Gene-specific therapies have recently been tried. Initial therapy of choice for symptomatic and also asymptomatic children is administration of beta-blockers.