Background: Rapid reperfusion of ST elevation myocardial infarction (STEMI) patients with primary percutaneous coronary intervention (PCI) improves outcomes. Systems have been developed in many areas to identify STEMI patients in the field using pre-hospital electrocardiograms (ECGs). The Zoll E-Series (ES) is a defibrillator/monitoring system that utilizes the GE Medical Systems 12SL interpretive algorithm to assist with identification of STEMIs. There is a lack of significant data measuring the accuracy of the Zoll ES and its algorithm, especially in the setting of a large urban population. Methods: We retrospectively looked at 307 consecutive ECGs transmitted by one emergency medical service (EMS) provider to an urban primary PCI facility from October 2009 to January 2011. All transmissions were from Zoll ES monitors. Multiple ECG computer interpretations were screened and filtered out to those labeled ***ACUTE MI***. A retrospective chart review was performed to determine the clinical outcome for each transmission in order to determine the performance of the interpretive algorithm in this population. Results: Overall, 307 ECGs were analyzed. A total of 7% (20 of 307) were confirmed STEMIs (Table 1). The computer reading shows a sensitivity of 95% (20 of 21) and specificity of 94% (268 of 286). There is a positive predictive value (PPV) of 53% (20 of 38). Conclusions: The initial data from this study shows that the Zoll ES has a remarkably high sensitivity of 95% but a poor PPV of 53%. This may be due to multiple confounders that result in STEMI “mimics.” The Zoll ES helps provide a pre-hospital ECG that serves as a powerful and life saving tool in the field. However, due to the low PPV, these findings reinforce that the pre-hospital ECG needs to be combined with an appropriate clinical history and additional human interpretation. Further studies analyzing a larger population of patients should be conducted to further assess the value of using this tool.