The consequences of environmental heat stress include a reduction in safety due to impaired concentration, a decreased work capacity, and heat illness. Maintaining adequate hydration is the single most important strategy to counteract the effects of thermal stress. In this study, the hydration status of groups of outdoor workers at mine sites and related facilities was assessed. Urine specific gravity was used as an indication of hydration levels. Fluid intake was monitored and fluid balance studies were carried out to assess the rate of sweat loss. The majority of workers were found to be inadequately hydrated. Most were hypohydrated at the commencement of the shift, and fluid intakes were, in general, well below those required to replace fluid losses — let alone improve hydration. Recommendations are given for maintaining adequate hydration of workers in hot conditions.