There have been no studies of the nature of defibrillatory shocks or risks to persons, providing them other than one individual case report and a number of incidental accounts. In this study, the severity and nature of injuries to prehospital emergency personnel in King County, Washington are reported. In addition, the types of injuries occurring to defibrillator operators throughout the United States, as voluntarily reported to the Food and Drug Administration (FDA), are described. In King County, prehospital emergency personnel reported eight accidental shocks. One individual was admitted to the hospital for 3 days and required lidocaine for premature ventricular contractions. Most injuries were the result of accidental contact with the patient rather than equipment failure, but the most serious case was caused by equipment failure. There were 13 injuries reported to the FDA over a 3 1/2-year period, and most injuries involved a mild shock or burn. Three patients were admitted to the hospital for observation. Two cases involved equipment failure. The rate of injury for paramedics was 1 per 1,700 defibrillatory shocks, and the rate of injury for emergency medical technician-defibrillator personnel was 1 per 1,000 defibrillatory shocks. These rates probably overestimate the real risk. Emphasis on safety and incorporation of safety procedures into resuscitation protocols can make the rate of injury even lower.