Resting electrocardiogram is a basic diagnostic tool used to identify myocardial diseases caused by a genetic defect of ion channels that results in bioelectric instability of cardiomyocytes and the risk of a complex induction of life-threatening ventricular arrhythmia. Long QT syndrome is the most common genetic ion channel disease affecting the general population. Congenital long QT syndrome is a heterogeneous group of ion channel genetic diseases, which manifest as a prolongation of QT interval on resting electrocardiogram. Some of the patients with long QT syndrome show QTc interval within the normal range, which is associated with the risk of false negative diagnosis. The use of function or pharmacological tests in patients with nondiagnostic QTc interval, but with a justified suspicion of long QT syndrome may provoke manifestation of phenotypic features of channelopathies. Prevention of cardiac events in patients with long QT syndrome involves, among others, the use of beta blockers. Implantation of a cardioverter-defibrillator is recommended in patients who developed cardiac arrest, and left cervicothoracic sympathectomy is also recommended in some cases. In terms of everyday functioning, patients with long QT syndrome should avoid drugs that prolong QT interval and have their electrolyte abnormalities corrected. Participation of long QT syndrome patients in competitive sports is still a matter of debate among the experts. The second part of the study, which is devoted to the prolonged QT interval on the electrocardiogram, focuses on provocation tests (both function and pharmacological tests) used in the diagnostics of long QT syndrome. Methods of risk stratification for sudden cardiac events in patients diagnosed with long QT syndrome, current possibilities associated with prevention and treatment as well as recommendations related to sports participation are also discussed.