The automatic implantable cardioverter-defibrillator (ICD) is highly effective in reducing sudden death rates in patients with life-threatening ventricular tachyarrhythmias. However, the magnitude of the ability of the ICD to improve overall survival is less certain. Data supporting the contention that the ICD prolongs survival are reviewed. It is evident that the mortality benefit consequent to the marked reduction in sudden death varies widely across subpopulations in a predictable manner. This observation reflects the powerful influence of other clinical factors that constrain survival in typical ICD patients. The implications for future studies on the ICD are discussed. (Circulation. 1994;89:1851-1858.)