Cooper ES, Berry MP, McMurray RG, Hosick PA, Hackney AC. Core temperature influences on the relationship between exercise-induced leukocytosis and cortisol or TNF-α. Aviat Space Environ Med 2010; 81:460–6. Introduction: The extent to which exercise in the heat modifies leukocytosis and the relationship between the leukocytosis and tumor necrosis factor-alpha (TNF-α) or cortisol is not well understood. Thus, this study attempted to determine the combined effect of exercise and differing elevations in core temperature on exercise-induced leukocytosis and to examine associations between any leukocytosis and cortisol or TNF-α. Methods: Eight male subjects completed two 40-min trials while immersed in 25 and 38.5°C water. Leukocytes, TNF-α, and cortisol were determined at baseline, immediately post-, and 2 h post-exercise. Results: Both trials resulted in significant 13–33% increases in total leukocytes, mostly driven by a 24–30% increase in neutrophils. A significant relationship was found between the change in core temperature (T_re) during exercise and the increase in total leukocytes (r = 0.561). Similarly, the change in T_re was related to the change in cortisol (r = 0.557) and TNF-α (r = 0.483). Yet the exercise-induced change in cortisol was not significantly correlated to any changes in leukocytes. There was a trend for the relationships between the exercise-induced change in TNF-α and the changes in total leukocytes (r = 0.491) and neutrophils (r = 0.479). Discussion: These results suggest that although neither cortisol nor TNF-α are strong predictors of the leukocyte response during exercise or recovery, each factor may be one of many potential modifiers of the total leukocyte response.