Although heat illness is recognized as a leading cause of death and disability during participation in U.S. high school and collegiate athletics, the incidence of heat illness among younger children and adults participating in sports and recreational activities is unknown. To examine the incidence and characteristics of nonfatal sports and recreation heat illness among persons of all ages, CDC analyzed 2001-2009 data from the National Electronic Injury Surveillance System--All Injury Program. This report describes the results of that analysis, which found that an estimated 5,946 persons were treated in U.S. emergency departments (EDs) each year for a heat illness sustained while participating in a sport or recreational activity, for an estimated annual rate of 2.0 ED visits per 100,000 population. Incidence was highest among males (72.5%) and among those aged 15-19 years (35.6%), and 7.1% of patients were hospitalized. These findings highlight the need for effective heat illness prevention messages to target all persons who are physically active, including those who participate in unstructured sports and recreational activities. Specific emphasis should be placed on targeting appropriate prevention messages toward those aged 15-19 years, who are at greatest risk, and their coaches and parents.