Nine patients were diagnosed with LQT1 and nine with LQT2. The other six individuals were healthy, with no symptoms characteristic for prolonged QT syndrome, but came from families with confirmed disease occurrence. The study was conducted on members of four families. In order to search for mutations (using mSSCP and sequencing), genomic DNA was obtained from patients to determine the expression levels of the genes KCNQ1 and KCNH2 (HERG), involved in the occurrence of clinical signs of disease.