This study assessed the energetic status of soldiers exposed to intense physical activities in cold and warm weather. Thirty subjects participated in a two-phase study: group A (n = 18) in the winter phase and group B (n = 12) in the summer phase. Energy expenditure (EE) was measured by the doubly labeled water technique; after a single, oral dosing of 2H218O, daily urine samples were collected for 12 successive days. Energy intake (EI) was assessed from detailed food records analyzed by computerized food charts. Energy balance was calculated as the difference between EI and EE for each subject. Mean (±SE) daily EE was 4,281 ± 170 and 3,937 ± 159 kcal/day for the winter and summer groups, respectively. Daily EI was 2,792 ± 124 kcal/day in group A and almost identical in group B. A negative energy balance of 1,422 ± 163 kcal/day and 924 ± 232 kcal/day (not significant) was calculated for groups A and B, respectively. Energy expenditure is primarily determined by the level of activity rather than by climate conditions; EI is insufficient to offset the high energy requirements under these conditions.