Long QT syndrome (LQT) is an inherited cardiac disorder that causes syncope, seizures and sudden death from ventricular tachyarrhythmias. We used single-strand conformation polymorphism (SSCP) and DNA sequence analyses to identify mutations in the cardiac sodium channel gene, SCN5A, in affected members of four LQT families. These mutations include two identical intragenic deletions and two missense mutations. These data suggest that SCN5A mutations cause LQT. The location and character of these mutations suggest that this form of LQT results from a delay in cardiac sodium channel fast inactivation or altered voltage-dependence of inactivation.