Exercise performed in hot or humid conditions can result in hyperthermia, which in turn can impair exercise performance, (1) but more importantly, it can result in exertional heat illness. (2,3) To reduce core temperature ([T.sub.C]), various cooling methods have been assessed, (4-17) including iced gastric and peritoneal lavage, pharmacologic-induced cooling, ice-pack application, evaporative cooling, and water immersion. Many of these methods provide efficient cooling rates, which are proposed (18) to be in excess of 0.2[degrees]C/min. Immediate cold-water immersion is the generally accepted "gold standard" technique for rapidly reducing [T.sub.C] when exertional heat stroke is suspected (ie, core temperature [greater than or equal to] 40[degrees]C). (16,17,19) When hyperthermia is mild (ie, core temperature slightly higher than 38[degrees]C), (20) other cooling methods may reduce the risk of exertional heat illness and provide relief to the athlete, allowing for a more rapid return to normothermia. Sensations of coolness associated with reductions in skin temperature may also offer some performance benefits to the athlete. (21) In these situations, the method chosen should be based primarily on effectiveness in reducing [T.sub.C], as well as on convenience and practicality. Cooling jackets are a popular method for addressing mild hyperthermia, offering convenience and practicality, particularly in field sport situations in which water, equipment, or power sources may not be available for cold-water immersion or convection cooling. To date, only 2 studies (21,22) have assessed the effects of wearing cooling jackets after an exercise bout performed in hot conditions. Although greater reductions in [T.sub.C] associated with wearing cooling jackets compared with a control condition were noted in both studies, only Webster et al (21) reported significant differences.