Electrophysiologic testing is known to be of value both diagnostically and in the evaluation of treatment modalities for patients with recurrent sustained ventricular tachycardia, out-of-hospital cardiac arrest, and syncope of unknown etiology. Attention is being focused on the possibility of identifying patients at high risk for such lethal ventricular arrhythmias in the hope that prophylactic therapy could prevent such arrhythmias from occurring. In this article, the authors discuss the potential role of electrophysiologic testing in this prospective identification and review the current data in the two groups of patients that have been studied extensively in this regard--post-myocardial infarction patients and patients with left ventricular dysfunction and congestive heart disease. The element of study artifact is also addressed.