The automatic implantable cardioverter-defibrillator continuously monitors the heart, identifies malignant ventricular tachyarrhythmias and then delivers electrical countershock to restore normal rhythm. There are two defibrillating electrodes which are also used for waveform analysis; one is located in the superior vena cava and the other is placed over the cardiac apex. A third bipolar right ventricular electrode is used for rate counting and R wave synchronization. When ventricular fibrillation occurs, a 25 J pulse is delivered; when ventricular tachycardia faster than the preset rate is detected, the discharge is R wave-synchronized. The clinical evaluation study of this therapeutic method began in February 1980 in patients with recurrent refractory life-threatening ventricular tachyarrhythmias. So far, the device has been implanted in nearly 500 patients with a follow-up period of up to 59 months. The risks and complications associated with this treatment were found to be moderate. Actuarial analysis has demonstrated significant impact on the survival rate of the patients receiving implants with 1 year arrhythmic mortality rate reduced to 2% or less in all groups analyzed. The available data indicate that the automatic cardioverter-defibrillator can reliably identify and correct potentially lethal ventricular tachyarrhythmias, leading to a substantial improvement in survival in properly selected high risk patients.