OBJECTIVE After activating 9-1-1 for out-of-hospital cardiac arrest (CA), guidelines for children 1 year and older have evolved to include immediate automated external defibrillator (AED) use for witnessed arrest, and two minutes of cardiopulmonary resuscitation (CPR) followed by AED use for unwitnessed arrests. The best approach to resuscitation in a two-tiered emergency medical services (EMS) system depends in part on how likely the patient is to present with ventricular fibrillation (VF). Therefore, the authors evaluated the frequency of VF with respect to age and other characteristics to further elucidate the role of the AED among pediatric CAs. METHODS The investigation was a retrospective cohort study of EMS-treated, nontraumatic, out-of-hospital CA among persons aged 1-18 years in King County, Washington, between April 1, 1976, and December 31, 2003. The primary goal was to identify the proportion of patients presenting to EMS in VF, according to age. The association between other characteristics and the likelihood of VF was also evaluated. Finally, hospital survival according to cardiac rhythm at EMS arrival was evaluated. RESULTS Ventricular fibrillation was the presenting rhythm in 17.6% of cases (48/272). The proportion presenting with VF was 7.6% (10/131) among children aged 1-7 years and 27.0% (38/141) among children aged 8-18 years (p < 0.001). In multivariable models, VF was independently associated with age 8 years and older compared with 1-7 years (odds ratio, 3.19; 95% confidence interval [CI] = 1.46 to 6.97), witnessed arrest (odds ratio, 3.33; 95% CI = 1.63 to 6.82), and cardiac etiology (odds ratio, 2.89; 95% CI = 1.32 to 6.34). Survival was 31.3% (15/48) for VF and 10.7% (24/224) for nonshockable rhythm CAs. CONCLUSIONS The proportion of children aged younger than 8 years presenting with VF is low compared with older children. The greatest increase in VF proportion occurs in children older than 12 years. Based on these results, the best approach for initial EMS resuscitation in a two-tiered EMS system, CPR versus AED use, is uncertain among younger children. Inclusion of witness status into the decision process for younger children may more efficiently allocate AED use, a finding in accordance with 2005 guidelines.