Structural firefighting is a highly stressful occupation with firefighters performing intense bouts of physical activity in environmental extremes while wearing impermeable, heavy and restrictive personal protective equipment. The aim of this study was to investigate the impact of performing occupational tasks during an active structural fire on firefighters’ hydration status. Nine fully qualified firefighters (mean ± SD age = 39.22 ± 7.89 years) completed a 15 min ‘live’ fire scenario while performing occupational tasks. Urine Specific Gravity (USG), body weight and tympanic membrane temperature were measured pre-scenario and at 0 and 20 min post-scenario. There was a significant decrease in body weight (0 min and 20 min p < 0.0005) and increase in tympanic membrane temperature (0 min and 20 min p < 0.0005) following the fire scenario. There was no significant change in USG post-scenario. Short duration firefighting operations can cause significant fluid loss, as measured by change in body weight but not necessarily USG.