The congenital long QT syndrome 3 (LQT3) is characterized by delayed cardiac repolarization, leading to a prolonged QT interval in the ECG. Common symptoms are syncopes, seizures, toursade-de-pointes arrhythmias and sudden cardiac death by ventricular arrhythmias. The LQT3 syndrome is caused by gain-of-function mutations in the α-subunit of the voltage-gated cardiac sodium channel SCN5A (Nav1.5). We found several novel mutations in the SCN5A gene associated with LQT3. Most of the affected residues are localized in the cytoplasmic domains. We functionally characterized the Nav1.5 mutations using the two-electrode voltage-clamp technique after expression in Xenopus oocytes. Using standard recording conditions, we measured Na⁺ current amplitude, voltage dependence of activation and inactivation, as well as the inactivation kinetics. We found that the most prominent gain-of-function mechanism of the novel mutations is an increase of current amplitude, while the persistent current was mostly not affected. In some mutants a reduced speed of inactivation or an enhanced recovery from inactivation contributed to the gain-of-function of the sodium currents. We conclude that the major cause of the LQT3 syndrome in our subset of mutations, is an increased current amplitude which was previously not described as a predominant mechanism of LQT3.