The effects of disease states and therapeutic drugs on the QT interval have been extensively studied in an attempt to understand the relationship between QT and the risk of torsade de pointes and sudden cardiac death. Differences in heart rate correction methods, electrocardiogram lead placement, and other internal (eg, genetic, physiologic) and external (eg, food, time of day) factors have confounded the interpretation of this relationship. A comprehensive review of the epidemiologic literature suggests that the corrected QT interval (QTc) is an important but imprecise marker of cardiovascular disease. The association between QTc prolongation and mortality has been identified in patients with cardiac disease but is unclear in patients without cardiac disease. Drug-related prolongation of QTc can clearly increase the risk of torsade de pointes, but this arrhythmia is rarely associated with a QTc of less than 500 ms. It also appears that noncardiac drugs that are associated with QTc prolongation are not identical in their proarrhythmic capacities and that increased exposure via clinically significant drug interactions is a major contributor to the liability of noncardiac drug-induced arrhythmia. Recognition of the aforementioned variables in conjunction with careful QTc measurements assists in establishing a more precise benefit-risk ratio for a specific drug therapy or for arrhythmia risk associated with various pathophysiologic or genetic states.