Sudden cardiac death is a massive public health problem that claims almost a half million lives a year in the United States. Several high-risk groups have been identified: (1) survivors of cardiac arrest; (2) patients with recurrent sustained ventricular tachycardia; (3) survivors of myocardial infarction (MI) who have more than 10 ventricular premature depolarizations (VPD) per hour or repetitive VPD; and (4) patients with class III or IV congestive heart failure (CHF) and unsustained ventricular tachycardia. The effects of open label drug treatment have been tested in survivors of cardiac arrest or sustained ventricular tachycardia using either electrophysiologic studies or 24-hour ECG recordings and exercise testing. Patients classified as drug responders using these methods have a lower mortality rate during follow-up than do those classified as nonresponders. This result is difficult to interpret. The test procedures may be identifying effective drugs, or alternatively, drug testing may identify high- and low-risk groups even though the drugs have no effect on survival. Recently, study designs have been proposed for conducting controlled trials in patients with malignant arrhythmias.