The long QT syndrome is characterized by prolongation of the heart rate-corrected QT interval on the electrocardiogram and by the occurrence of life-threatening ventricular tachyarrhythmias of the torsade de pointes type. In most cases, the disorder becomes clinically manifest during childhood. Before these events, affected patients are typically considered healthy because no other symptoms occur. Sudden cardiac death without preceding syncope is rare. The diagnosis is based on the patient's history and the evaluation of the surface electrocardiogram. Genetic analysis is a relatively new diagnostic tool. The disease-causing gene can be identified in approximately 70-80% of patients. Symptomatic patients are treated with a beta-blocker. Implanted cardioverter defibrillators appear to be the most effective therapy for high-risk patients. High-risk patients are defined as those with aborted cardiac arrest or recurrent cardiac events (e. g., syncope or torsade de pointes) despite conventional therapy (i. e., beta-blocker alone).