Patients with congestive heart failure have a high incidence of sudden cardiac death that is attributed to ventricular arrhythmias. The mortality rate in a group of patients with class III and IV heart failure is about 40% per year, and half of the deaths are sudden. Half of the patients with New York Heart Association class III or IV heart failure have unsustained ventricular tachycardia detected on a 24 hr continuous electrocardiographic recording. The presence of ventricular tachycardia in patients with congestive heart failure increases the probability of dying; in class III or IV heart failure, the presence of unsustained ventricular tachycardia on a 24 hr continuous ECG recording increases the odds of dying about threefold over a 1 to 2 year follow-up period. Many electrical, mechanical, humoral, and electrolyte abnormalities may promote ventricular arrhythmias in patients with heart failure. Correction of these predisposing factors could reduce the risk of lethal ventricular arrhythmias and therefore every effort should be made to do so. Because there has been no definitive study of the impact of antiarrhythmic drug treatment on the survival of patients with heart failure and ventricular arrhythmias, the role of therapy with antiarrhythmic drugs remains uncertain at the present time.