The measurement of energy expenditure has wide applications in clinical and scientific studies. Ethical and practical problems, however, have limited the acquisition of information on total energy expenditure in infancy. The doubly-labelled-water technique, recently validated for use in infants, has now been used to measure, non-invasively, total energy expenditure in a cohort of forty-one normal, full-term infants at or close to 1.5, 3 and 6 months of age. Mean total energy expenditure was 270, 280 and 330 kJ/kg per d at these ages. Centiles for total energy expenditure in early infancy are presented; it is suggested such data are most appropriately expressed as kJ/square root (kg body-weight per d). These findings will be of importance in the re-evaluation of energy requirements in infancy and in the study of lesions in energy metabolism in disease states at this age.