To quantify the magnitude of dehydration and to identify predictors of dehydration by utilizing changes in total body water (TBW), 24 male Marines were studied during an 8d moderately cold-weather (1-3 degrees C) training exercise. Training consisted of approximately 12 h/d of moderately intense activity. Variables examined included body composition, TBW, and various blood and urinary parameters indicative of hydration. Dietary food and fluid records were also maintained. Body composition changes were consistently with the high energy expenditure and insufficient energy intake. Despite a decrease in TBW, significant dehydration as assessed by blood and urinary parameters did not occur over the 8d period, therefore no relationships were found between TBW and any of the clinical indicators of dehydration. The change in TBW was reflective of a decrease in lean body mass. These data suggest that during cold-weather military field exercise, despite high activity levels and a negative energy balance, body fluid balance can be maintained when particular attention is paid to fluid intake.