AbstractThe energy cost of physical activity has long remained the component of energy expenditure most difficult to quantify. Measurement of total energy expenditure (TEE) in free‐living infants by the doubly‐labelled water method permits a new approach. The energy cost of activity was estimated in 12 week infants as the difference between TEE and the sum of minimal metabolic rate, the energy cost of tissue synthesis, and thermogenesis. No differences were found between the sexes or between breast‐fed and formula‐fed infants. However, the cost of activity was substantially higher than previously assumed. The approach is more suitable for use with groups rather than individuals. © 1995 Wiley