Purpose: Victims of Out-of-Hospital Cardiac Arrest (OHCA) present, in about 20% of cases, with ventricular fibrillation (VF) treated with an Automated External Defibrillator (AED) used by the attending emergency service personnel. The aim of this study was tomeasure the efficiency of AEDsusedbyprofessional rescuers in an urban area, by observing the post-shock electrocardiographic tracing and their time course after the first External Electric Shock (EES). Materials andmethods:Prospectiveobservational study. Inclusion criteria: OHCA victims>18 years of age with VF treated by EES/AED. Data collected: AED rhythm: 5, 15, 30, and 60 s post-shock. The primary endpoint was the resolution of VF at 5 s post-shock. Secondary endpoint was occurrence of Organized Rhythm (OR) on patients that had been shocked. ORwas defined by thepresenceof at least 2electrocardiographicorganizedcomplexes in an interval of 10 s. Results: From 01/06/11 to 01/09/11, 111 consecutive OHCA treated by EES/AED were analyzed. Mean age was 66+20 years, 77% were men, and 32% occurred in public place. The average time between cardiac arrest and 1st shock was 10+3min. The results are shown in Fig. 1 (90% efficiency at 5 s). Out of the 100 patients fulfilling the primary endpoint, 41 experienced VF recurrence in post-shock atminute 1. VF recurrencewas related to asystolemore than to organized rhythm (48% (n=34) vs. 10% (n=3), respectively; p=0.001).