Four classifiers were evaluated to determine differences in case detection for heat related illness. The results of those classifiers, as applied to data from Northern Illinois ESSENCE participating hospitals from May 1, 2012 through August 31, 2012, will be discussed. Three refined syndrome definitions for heat related illness (chief complaint-based, discharge diagnosis-based, and both chief complaint and discharge diagnosis) were created based on the four evaluated classifiers and these classifiers were applied to current 2013 data. The application of the syndrome definition is dependent on a jurisdiction''s purpose for conducting heat related illness surveillance.