Undertaking operational patrolling activity in hot environments, carrying heavy loads and wearing personal protective equipment (PPE) presents a significant physiological challenge to Service personnel. Planned research trials involving volunteers exercising in the heat observe an important volunteer withdrawal criterion of attainment of a core body temperature of 39.5 °C. However, trained populations (marathon runners) have been shown to sustain exercise with core temperatures in excess of 40 °C with no recognised adverse health effects [1]. Thus, it could be hypothesised that military personnel with a good level of physical fitness might similarly sustain higher than usual core temperatures, albeit while wearing impermeable PPE. The aim of the present study was to measure the actual thermal response of military personnel while performing operationally specific exercises in the most arduous phase of a military exercise in a hot-dry environment.