Study objective: Successful resuscitation of cardiac arrest depends partly on the time of first defibrillation. An accurate, practical method of measuring this time has not been devised. We attempted to determine the interval from receipt of a call by emergency medical services personnel to first defibrillation (total defibrillation interval) with synchronized clocks between computer-aided dispatch operations and an event-recording defibrillator. Design: A 7-month prospective study measuring the total defibrillation interval. An automated code summary was to be submitted for each participant. Setting: An urban, all–advanced life support, public utility model system. Participants: All primary ventricular fibrillation patients seen during the study period. Results: Ninety-two patients met study criteria. Data are presented as median (interquartile range). The total defibrillation interval was 9.8 minutes (7.9 to 11.8 minutes). The call-receipt–to–vehicle-at-scene interval was 5.98 minutes (4.4 to 7.3 minutes). The vehicle-at-scene–to–defibrillation interval was 3.6 minutes (2.5 to 4.6 minutes). Conclusion: The use of synchronized clocks in automated event-recording systems may provide a method of accurately measuring the time elapsed before defibrillation. [Campbell JP, Kroshus KS, Lindholm DJ, Watson WA: Measurement of the call-receipt–to–defibrillation interval: Evaluation of prehospital methods. Ann Emerg Med December 1995;26:697-701.]