By the combination of energy and macronutrient balances, continuous open circuit computerized indirect calorimetry, and anthropometry, we have compared small for gestational age (SGA) and appropriate for gestational age (AGA) very low birthweight infants with respect to metabolizable energy intake (mean ± SE: 125.9 ± 2.5versus130.4 ± 3.5 kcal/kg±day), energy expenditure (67.4 ± 1.3versus62.6 ± 0.9 kcal/kg±day), storage of energy and macronutrients and growth. Fourteen studies in six SGA infants (gestational age, 33.1 ± 0.3 weeks; birthweight, 1120 ± 30 g) and 22 studies in 13 AGA infants (gestational age, 29.3 ± 0.4 weeks; birthweight, 1155 ± 40 g) were performed. The SGA infants had a lower absorption of fat (68.7 ± 3.2versus79.7 ± 1.7%) and protein (69.1 ± 3.2versus83.4 ± 1.5%) and hence increased (P> 0.001) energy loss in excreta (29.9 ± 2.8versus18.2 ± 1.5 kcal/kg±day). The significant hypermetabolism of SGA infants by 4.8 kcal/kg±day was associated with an increased fat oxidation. Despite lower energy storage, SGA infants were gaining weight (19.4 ± 0.9 g/kg±day), length (1.25 ± 0.14 cm/week), and head circumference (1.16 ± 0.9 cm/week) at higher rates than the AGA group. The energy storage per g weight gain was lower (P> 0.001) in the SGA group (3.0 ± 0.14versus4.26 ± 0.26 kcal) reflecting higher water, lower fat (22.2 ± 1.8versus33.8 ± 2.5%;P> 0.001) and lower protein (7.7 ± 0.5versus12.5 ± 0.8%;P> 0.001) contents of weight gain in the SGA group.