Heat illness during practice or competition is a leading cause of death and disability among U.S. high school athletes. An estimated 7.5 million students participate in high school sports annually. To examine the incidence and characteristics of heat illness among high school athletes, CDC analyzed data from the National High School Sports-Related Injury Surveillance Study for the period 2005-2009, which includes the 2005-06, 2006-07, 2007-08 and 2008-09 school years. During 2005-2009, the 100 schools sampled reported a total of 118 heat illnesses among high school athletes resulting in >or=1 days of time lost from athletic activity (i.e., time-loss heat illness), a rate of 1.6 per 100,000 athlete-exposures and an average of 29.5 time-loss heat illnesses per school year. The average corresponds to a weighted average annual estimate of 9,237 illnesses nationwide. The highest rate of time-loss heat illness was among football players, 4.5 per 100,000 athlete-exposures, a rate 10 times higher than the average rate (0.4) for the eight other sports. Time-loss heat illnesses occurred most frequently during August (66.3%) and while practicing or playing football (70.7%). No deaths were reported. Consistent with guidelines from the National Athletic Trainers' Association (NATA), to reduce the risk for heat illness, high school athletic programs should implement heat-acclimatization guidelines (e.g., set limits on summer practice duration and intensity). All athletes, coaches, athletic trainers, and parents/guardians should be aware of the risk factors for heat illness, follow recommended strategies, and be prepared to respond quickly to symptoms of illness. Coaches also should continue to stress to their athletes the importance of maintaining proper hydration before, during, and after sports activities.