RESUMEN
Prehospital Emergency Medical System is one of the critical parts in health care sector which has been usually unnoticed. Delivering quick, efficient and high quality care in this system has a large impact on mortality and morbidity of patients. In this descriptive and cross-sectional study, 20662 cases in 4 months in the middle of the year 2008. Missions over 8 minutes from when the time of call receiving to arrival on the scene were detected and survived For statistical analysis ANOVA was used. Most of the missions were done between 8 to10 minutes. Nine reasons were detected for this arrival delay:1] destruction of the road, 2] Traffic, 3] Rainy weather, 4] Long way to the scene, 5] Scarcity of ambulance in the center near the scene, 6] Wrong address, 7] Cannot find the address, 8] Delay in dispatching the ambulance, 10] Delay in arrival because of technicians. In Shiraz Emergency Medical system the most important reasons for missions over 8 minutes were scarcity of ambulance in the center near the scene, traffic, long way to the scene. Quick arrival on the scene has a large effect on patients, though, health ministry and emergency centers should pay more attention on this sector and deliver financial support for equipped emergency centers due to the population and demand for emergency centers.