Congenital long QT syndrome is an inherited disorder of cardiac repolarization that predisposes to syncope and to sudden death from polymorphic ventricular tachycardia. The disorder should be suspected when the electrocardiogram shows characteristic QT abnormalities, or when there is a family history of long QT syndrome or of an event that raises suspicion of long QT syndrome, such as sudden death, syncope, or ill-defined "seizure" disorder. We can now classify some types of congenital long QT syndrome according to their genetic mutations and their triggers, such as exercise, rest, or startle.