The long QT syndrome (LQTS) is an inherited cardiac arrhythmia that may lead to sudden death in the absence of structural heart disease. Mutations in the cardiac potassium and sodium channel genes can be found in approximately 70% of patients with a highly probable clinical diagnosis. In this study, we aimed to genotype and explore the yield of genetic testing of LQTS patients from Greece, for whom there are no collective published data available. We clinically evaluated and genetically screened 17 unrelated patients for mutations in the KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2 cardiac ion channel genes. Genetic testing was positive in 6 out of 8 patients with a highly probable clinical diagnosis of LQTS and negative for all the other patients. Two patients carried KCNQ1 mutations (c.580G>C, c.1022C>T), while 4 patients carried KCNH2 mutations (c.202T>C, c.1714G>A, c.3103delC, c.3136C>T). To the best of our knowledge, the last mentioned mutation (c.3136C>T) is novel. Moreover, 27 single-nucleotide polymorphisms (SNPs) were detected, 5 of which are novel. Our preliminary data indicate a low genetic diversity of the Greek LQTS genetic pool, and are in accordance with international data that genetic testing of the major LQTS genes is efficient in genotyping the majority of patients with a strong clinical diagnosis. Therefore, the transition of an LQTS genetic screening program from research to the diagnostic setting within our ethnic background is feasible.