Clinical, angiographic, echocardiographic and electrophysiologic data were examined in 101 patients with a history of sustained ventricular arrhythmia not associated with acute myocardial infarction. These patients included 66 survivors of out of hospital cardiac arrest and 35 patients presenting with hemodynamically well tolerated sustained ventricular tachycardia. On univariate analysis, patients in the cardiac arrest group had a lower incidence of previous myocardial infarction and left ventricular aneurysm and a higher ejection fraction compared with the ventricular tachycardia group. During electrophysiologic testing, the arrhythmia induced in the patients in the cardiac arrest group was fast and polymorphic and frequently degenerated into ventricular fibrillation. In contrast, in the ventricular tachycardia group, a slower, monomorphic and hemodynamically well tolerated ventricular tachycardia was commonly induced. On multivariate analysis, a polymorphic pattern of the induced ventricular arrhythmia was the only independent variable that distinguished the survivors of cardiac arrest from those presenting with sustained ventricular tachycardia. These results suggest that 1) the survivors of cardiac arrest and patients presenting with sustained well tolerated ventricular tachycardia are clinically distinct groups; and 2) the polymorphic tachycardia induced during programmed electrical stimulation in the survivors of cardiac arrest may indicate an unstable tachycardia mechanism. This may explain why these patients present with ventricular fibrillation and cardiac arrest, whereas others present with hemodynamically stable ventricular tachycardia.