This study investigates the effects of exertional heat stress and acclimation status on physiological and cognitive performance. Forty male soldiers performed an exertional heat stress test (EHST) either in a cool (20 degrees C, 16 degrees C wet bulb globe temperature), or in a hot environment (40 degrees C, 29 degrees C wet bulb globe temperature), unacclimatized, or after 10 days of passive or active acclimation. Mean skin and tympanic (Tty) temperatures and heart rates (HR) measured physiological strain. A cognitive test (the computerized Cambridge Neuropsychological Test Automated Batteries attention battery) is administered before and immediately after EHST. EHST in hot conditions induced physiological heat stress (increase in Tty and HR), which caused mild deficits in attention in U group (decreased number of correct responses, and prolonged movement time). Acclimated (passive and active) soldiers suffered no detrimental effects of exertional heat stress, despite almost the same degree of heat strain, measured by Tty and HR.