In order to test the efficacy of a semiautomatic defibrillator (Heartstart 2000) in connection with cardiac arrest outside hospital, the apparatus was installed in two of the ambulances belonging to the Copenhagen Fire Service. The ambulance district involved was also equipped with an ambulance staffed by a doctor. A total of 48 patients with cardiac arrest were found and 16 of these had ventricular fibrillation. Six of the 16 patients have since been discharged from hospital (37.5%). The defibrillator had a high diagnostic certainty with a sensitivity of 96.5% and a specificity of 100%. No practical problems of note occurred in connection with employment of the defibrillator. The ambulance staffs underwent six hours of training and this appeared to be adequate. It is concluded that Heartstart 2000 functions well and effectively in connection with revival of patients with cardiac arrest outside hospital.