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Five of 28,753 cyclists participating in an annual 109-km bicycle race died, four within 24 h of the race and the fifth 17 d later. All five deaths were reported to be the consequence of exertional heatstroke. One runner of 6874 participating in an annual 56-km ultramarathon developed heatstroke and required active cooling for 10 h to achieve normothermia. The purpose of this article was to postulate (i) why only 6 of 35,627 athletes were hospitalized for heatstroke in these races, (ii) if exercise alone could have elevated their body temperatures sufficiently to cause heatstroke, and (iii) why the runner required such prolonged cooling.Clinical and autopsy data are presented for three of the cyclists and the runner for whom access to this information was granted. Calculations were made to predict the work rates necessary to produce their measured rectal temperatures.The rectal temperatures of two of the cyclists were 42.0 and 41.2°C on hospitalization, and that of the runner was 41.8°C on collapse. Standard calculations showed that in the prevailing environmental conditions and with their exercise speeds, none should have developed exertional heatstroke. The third cyclist experienced a cardiac arrest to which his elevated (rectal) temperature may have contributed.The hyperthermic states experienced by the cases presented may have resulted from failure of their heat-losing mechanisms. Alternatively, they might have resulted from excessive endothermy, triggered by physical exertion and other unknown initiating factors. Excessive endothermy should be considered in cases of heatstroke that occur in mild to moderate environmental conditions. Furthermore, prompt initiation of cooling is crucial in all cases of suspected heatstroke.