The long QT syndrome (LQTS) combines a prolonged QT interval with an enhanced risk of polymorphous ventricular arrhythmias that may lead to syncope and sudden cardiac death. It may be congenital or acquired (the latter sometimes caused by drugs). Congenital LQTS is a rare disease, usually discovered during the clinical evaluation of understood syncopes or at cardiological examination after an unexpected sudden cardiac death of a close relative. The syncope frequently occurs during physical exercise, fear or sudden loud noises. In patients with symptomatic LQTS, the mortality 10 years after the first syncope amounts to approximately 50%. A prolonged QT interval indicates abnormal repolarization or deceleration of the depolarization. An increase of the sympathetic tone, e.g. during physical exercise and emotions, causes prolongation of the QT interval. Congenital LQTS has been associated with genetic mutations, for instance on chromosomes 3 and 7. Treatment consists af administration of beta-blockers, sympathectomy and, if necessary, implantation of an automatic cardioverter/defibrillator.