In the United States alone 300,000-400,000 people die of sudden cardiac death every year. Much of this mortality is assumed to be caused by ventricular tachyarrhythmias. Prolonged QTc reflect cardiac repolarization prolongation and/or increased repolarization inhomogenity known to be associated with increased risk of arrhythmias. The paper gives a review of the possibilities to assess the risk of ventricular arrhythmia and/or cardiac death from QTc. Prolonged QTc may hold independent prognostic importance for mortality in common diseases as ischemic heart disease and diabetes mellitus where as the prognostic importance in heart failure and arterial hypertension is more uncertain. In more rare diseases as the inborn long QT syndrome the QT interval gives not only important hint to the diagnosis but the magnitude also provides information on prognosis. QTc has probably no independent prognostic importance in hypertrophic cardiomyopathy or in the arrhythmogenic right ventricular disease. The degree of QTc prolonging during treatment with QTc prolonging drugs is prognostic for the risk of ventricular arrhythmia in form of torsade de pointes and QTc prolonging drugs should probably not be prescribed for patients with a QTc greater than 460 ms and withdrawn if QTc exceeds 500 ms during treatment. Data from the DIAMOND study suggest that QTc can be used to point out those heart failure patients who will benefit from antiarrhythmic therapy.