Congenital long QT syndrome (LQTS) is an inherited cardiac arrhythmia disorder characterized by prolongation of the QT interval; patients are predisposed to ventricular tachyarrhythmias and fibrillation leading to recurrent syncope or sudden cardiac death. We performed clinical and genetic studies in six Saudi Arabian families with a history of sudden unexplained death of children. Clinical symptoms, ECG phenotypes, and genetic findings led to the diagnosis of LQT1 in two families (recessive) and LQT2 in four families (three recessive and one dominant). Onset of arrhythmia was more severe in the recessive carriers and occurred during early childhood in all recessive LQT1 patients. Arrhythmia originated at the intrauterine stages of life in the recessive LQT2 patients. LQT1, causing mutation c.387-5 T > A in the KCNQ1 gene, and LQT2, causing mutation c.3208 C > T in the KCNH2 gene, are presumably founder mutations in the Assir province of Saudi Arabia. Further, all LQTS causing mutations detected in this study are novel and have not been reported in other populations.