Ventricular arrhythmias can be classified into categories based on the risk of sudden death associated with them. Patients with benign ventricular arrhythmias are those with minimal or no heart disease and normal ventricular function. Arrhythmias encountered in this group include ventricular premature complexes and unsustained ventricular tachycardia. In patients with prognostically significant ventricular arrhythmia, the arrhythmias are the same, namely, ventricular premature complexes and/or unsustained ventricular tachycardia. This group differs, however, in that the patients have organic heart disease and the risk of the arrhythmia increases as left ventricular function decreases. Patients with malignant ventricular arrhythmia, sustained ventricular tachycardia and ventricular fibrillation have the highest risk of sudden death. While no conclusive studies are available, some studies do suggest that the suppression of ambient ventricular arrhythmia as documented by Holter monitoring (and possibly exercise testing) identifies antiarrhythmic regimens which are effective in preventing sudden death. Variability in the frequency of ambient ventricular ectopy must be considered in assessing both efficacy and toxicity of antiarrhythmic regimens. Holter monitoring may also be useful in assessing the substrate of malignant ventricular arrhythmias and autonomic tone.