The Electronic Surveillance System for the Early Notification of Community-based Epidemics (ESSENCE), developed by the Department of Defense Global Emerging Infections System (DOD-GEIS), actively analyzes syndromic groupings from electronic International Classification of Diseases, Ninth Revision data as a proxy for early disease outbreak detection. This study compares International Classification of Diseases, 9th Revision, data and emergency room records from three hospitals to determine the accuracy of data in ESSENCE. Of 2474 records reviewed, inter-reviewer variability illustrated excellent consistency, ranging from 0.87 to 1.0. Gastrointestinal disease had the highest overall sensitivity (89.0%) and specificity (96.0%), likely due to less overlap with other groups, unlike the respiratory (sensitivity, 65.7%; specificity, 95.6%) and fever (sensitivity, 69.4%; specificity, 95.5%) groups, where symptoms of both are often seen in the same patient. This study concludes that data used by ESSENCE is accurate and reflects the types of patient visits to these facilities: valuable information for public health decision makers.