The antiarrhythmic principle of drug-induced QT-interval prolongation is well known. However, with the widespread use of the presently known and new Class III antiarrhythmic agents under investigation, and the growing number of agents not primarily designed as antiarrhythmic drugs that potentially cause QT prolongation, we have also become aware of the proarrhythmic hazards associated with many of these agents. The proarrhythmic risk differs markedly from one agent to another and interferes with many individual clinical variables (e.g., hypokalemia, sinus bradycardia). This paper summarizes the present data on the proarrhythmic risk of drug-induced QT prolongation, including the value and problems of the rate-corrected QT interval, the mechanisms involved in the genesis of proarrhythmia, and the clinical cofactors that facilitate the occurrence of proarrhythmic events. In addition, an extensive database provides information on the known proarrhythmic risk of all currently used QT-prolonging agents.