Traditionally, infant energy requirements have been predicted from body size or age, whereas in older children and adults, physical activity is also taken into account. However, the extent to which body size determines energy use in individual infants has not been considered. Data on 232 measurements of total energy expenditure obtained in 124 infants aged 1.5 to 12 months were used to assess the relation between body size and energy use in individuals. Age, weight, and fat free mass consistently predicted total energy expenditure with an error of 21-23%. This contrasts greatly with the error of 10% with which infant basal metabolism can be predicted from anthropometry. Body size is a poor index of the total energy requirements of individual infants, and predictive equations generated from data on healthy infants will be inappropriate for disease states where physical activity or growth is altered.