Ventricular fibrillation, an abnormal cardiac rhythm, occurs in at least twothirds of the 400,000 people who die out of the hospital from sudden cardiac arrest. This rhythm can be treated successfully by electric countershock, a procedure known as defibrillation. The survival rate following such cardiac arrest is directly related to the rapidity of response; the shorter the time from collapse to defibrillation, the more patients will survive. There are two basic options to shorten the time from collapse to defibrillatory shock. The first is to upgrade the emergency medical system. The second is to provide spouses and family members of potential cardiac arrest patients with automatic home defibrillators. This article considers the effectiveness of the second option, home defibrillation, compared with that of an equally costly upgrade in existing emergency medical service systems. The comparisons depend on the existing level of emergency medical service system, the cost of the home defibrillator, and the rate at which a home defibrillator would be used appropriately. The comparisons suggest that in many circumstances home defibrillation is an appropriate option to be considered.