CONTEXT: Recommendations for heat illness prevention provided by sports medicine associations do not always account for sex differences, specific age populations, regional environmental conditions, equipment worn during activity, or the athlete's size or preexisting level of fitness. OBJECTIVE: To evaluate the rate of exertional heat illness (EHI) among collegiate football athletes and to monitor environmental conditions during American football practice for a 3-month period. DESIGN: Epidemiologic study in which we reviewed the occurrence rates of EHI and wet bulb globe temperature readings during a 3-month period of American collegiate football practice sessions. SETTING: Five universities in the southeastern region of the United States. PATIENTS OR OTHER PARTICIPANTS: Collegiate football players at the 5 universities. MAIN OUTCOME MEASURE(S): Wet bulb globe temperatures were recorded from August through October 2003, at the beginning, middle, and end of each practice session. The EHIs were identified and recorded, and athlete-exposures (AEs) were calculated. RESULTS: A total of 139 EHIs and 33 196 AEs were reported (EHI rate = 4.19/1000 AEs). The highest incidence of EHIs was in August (88%, EHI rate = 8.95/1000 AEs) and consisted of 70% heat cramps (6.13/1000 AEs), 23% heat exhaustion (2.06/ 1000 AEs), and 7% heat syncope (0.58/1000 AEs). No cases of heat stroke or hyponatremia were identified. The highest risk of EHI occurred during the first 3 weeks of the study; mean wet bulb globe temperature declined significantly as the study continued ( P < .001). Temperatures in the final 5 weeks of the study were significantly cooler than in the first 5 weeks ( P < .05). CONCLUSIONS: Heat cramps were the most common EHI and occurred most often during the first 3 weeks of practice. Athletic trainers should take all necessary preventive measures to reduce the risk of EHI.