An automatic external defibrillator (AED) was used by paramedics to detect ventricular fibrillation and deliver countershocks in 39 people with out-of-hospital cardiac arrests. The AED identified and delivered at least 1 countershock to 13 of the 16 people in ventricular fibrillation (81% sensitivity). The AED responded correctly to all 21 of the non-ventricular-fibrillation rhythms (8 other electrical rhythms, 13 asystole) with no countershocks (100% specificity). In 2 patients the rhythm could not be assessed. The device caused no injuries to patients or personnel. The performance of the AED was also analysed by considering each 15 s segment of ventricular fibrillation as a separate challenge; the device delivered a countershock in 19 of 29 such segments (66%).