Patients who experience malignant ventricular arrhythmias (i.e., ventricular fibrillation or ventricular tachycardia with syncope or with hemodynamic compromise) are at high risk of sudden death. Such patients can now be protected from recurrent arrhythmias by the use of conventional and experimental drugs. Drug therapy must be individualized, and this requires a system of testing to expedite the selection of the most efficacious and least toxic agent. In 85% of these patients, the frequency and advanced grades of ventricular premature beats exposed either by Holter monitoring or by maximal exercise stress testing provide an adequate target for assessing drug action. Only 15% of patients require invasive electrophysiologic studies to guide antiarrhythmic therapy. In 10% of instances, antiarrhythmic drugs cause aggravation of arrhythmias. When drug therapy is individualized, an effective program can be achieved for 80%, with less than a 3% incidence of sudden death annually.