Prolongation of QT interval is an indicator of bad prognosis in general population of people with heart disease, in patients after myocardial infarction and in people without recognized heart disease. Prolongation of QT interval occurs in many diseases and clinical states. It may be congenital or acquired ECG anomaly and it determines the basic symptom of long QT syndrome. In this paper we present clinical meaning of QT interval, measurement rules and problems with correct valuation of QT/QTc in electrocardiogram. We also discuss reasons of QT interval prolongation--congenital or acquired long QT syndrome, as well as the procedure in case of QT interval prolongation and possibilities of preventive treatment of iatrogenic long QT syndrome.