This study was undertaken to assess to what extent estimated energy content of 13 infant formulas was related to gross energy measured by bomb calorimetry. Measured gross energy exceeded significantly metabolizable energy as claimed by the manufacturers by applying the classical Atwater's factors by about 12% (range, 6-17%). Measured gross energy also exceeded significantly gross energy calculated by applying either FAO/WHO or our own values of heat combustion of nutrient by 6% (range, 3-9%) and 3.5% (range, 1-6%), respectively. It is concluded that for reasons of symmetry and simplicity, the labeling of the energy content of infant formulas should continue to be calculated by applying the classical Atwater's factors to the nutrient composition. However, when accurate energy balance studies are carried out, the energy content of the diet should be measured by bomb calorimetry rather than be estimated by applying conversion factors to claimed or determined nutrient composition.