Distance runners compete in a variety of environments such as hot humid conditions for extended periods of time. Extended performance in hot humid environments negatively affects running performance. PURPOSE: The purpose of this study was to investigate the effects of a Cool Shirt™ worn by male and female distance runners training in a hot humid environment. METHODS: Fourteen distance runners participated in the study. Testing was performed in Human Performance Lab at the University of Louisiana at Monroe. Subjects VO2max was measured using open circuit spirometry on a treadmill ergometer. On two separate occasions participants returned to the lab for treadmill run in a hot humid environment at the speed and grade at which 85% of the subject's predetermined VO2max was maintained. Randomly assigned, each participant conducted a control run and a run utilizing a Cool Shirt™. Heart rate, Borg's Rate of Perceived Exertion, temperature of the auditory canal, and sweating rate were measured. Statistical analysis was conducted using SPSS 13.0. Significance level was preset at p<.05. RESULTS: No significant difference in RPE, HR, and run to exhaustion time was found between Cool Shirt™ runs and control runs. CONCLUSIONS: The materials that are used to create the Cool Shirt™ may be the largest factor that prevents the effective cooling of the user. The cotton material used may absorb sweat and contribute to more energy expenditure needed to support the weight of the shirt. More research is needed to determine if the weight of the shirt outweighs the possible benefits that it could possibly provide for users. In the current state of development, this product is not feasible for use during most training situations and competition because of its lack of portability by the individual user. With the development of a smaller portable storage tank and pump along with the development of a shirt with lighter, more technical materials, ergogenic effects could be possible. More research is needed in this area to confirm the feasibility, use and benefits. Cool Shirt™ use for a precooling protocol may be a more applicable and more efficient use of the current product. An area of future research with Cool Shirt™ could be to investigate use as a precooling agent and to develop precooling protocols for optimal performance.