We used two national injury surveillance systems to examine heat illnesses in two different U.S. population subsets. We used the National Electronic Injury Surveillance System-All Injury Program (NEISS-AIP) to examine heat illness incidence and characteristics among sports and recreation participants of all ages from 2001 to 2009, and we used the National High School Sports-Related Injury Surveillance Study (High School RIO TM ) to examine heat illness incidence and characteristics among high school-aged athletes from 2005 to 2009 (Table 1) . NEISS-AIP, operated by the U.S. Consumer Product Safety Commission, monitors consumer product-related injuries treated in a nationally representative sample of 66 U.S. hospital emergency departments (EDs) (3). Trained coders enter demographics, a brief narrative and consumer product information for each injury presenting to their ED. High School RIO TM , operated by the Center for Injury Research and Policy at Nationwide Children's Hospital (Columbus, OH), monitors sports injuries in a nationally representative sample of 100 high schools (4). Certified athletic trainers at participating schools report exposure and injury data electronically.