['The use of automatic external defibrillators (AEDs) by EMS initial responders is widely advocated. Evidence supporting the use of AEDs is based largely on the experience of one metropolitan area, with effect on survival in many systems not yet proved. We conducted this study to determine whether the addition of AEDs to an EMS system with a response time of 4 minutes for first-responder emergency medical technicians (FREMTs) and 10 minutes for paramedics would affect survival from cardiac arrest.', 'This prospective, controlled, crossover study (AED versus no AED) of consecutive cardiac arrests managed by 24 FREMT fire companies took place from 1992 to 1995 in Charlotte, North Carolina, a city of 455,000. Patients were stratified using the Utstein criteria. The primary endpoint was survival to hospital discharge among patients with bystander-witnessed arrests of cardiac origin.', 'Of the 627 patients, 243 were bystander-witnessed arrests of cardiac origin. Survival to hospital discharge was accomplished in 5 of 110 patients (4.6%; 95% confidence interval [CI] 0.6% to 8.4%) with AED compared with 7 of 133 (5.3%, 95% CI 1.5% to 9.1%) without AED (P = .8). Both groups were comparable with regard to age, gender, history of myocardial infarction, congestive heart failure or diabetes, arrest at home, bystander CPR, and whether or not ventricular fibrillation (VF) was the initial rhythm. For arrests of any cause, witnessed by bystanders or EMS personnel, with an initial rhythm of VF or ventricular tachycardia (VT), 5 of 77 (6.5%, 95% CI 1.0% to 12.0%) with AED survived compared with 8 of 105 patients (7.6%, 95% CI 2.5% to 12.7%) without AED (P = .8). Statistically significant differences were noted in race and EMS response times between the two groups, which did not affect survival.'] Does eMT defibrillation increase survival from sudden cardiac death in a two-tiered urban-suburban EMS system?