To compare the growth and accumulation of protein, fat, and carbohydrate in the formula-fed premature infant and in the fetus of a similar postconceptional age, we performed 22 metabolic studies in 13 infants of very low birth weight (1155±39 g [mean ±S.E.]). Measurements combining nutritional balance and indirect calorimetry demonstrated the deposition rates of protein and fat. We found that the formula-fed, very-low-birth-weight infant who gained weight comparably to the fetus retained the same amount of protein (1.92±0.1 g per kilogram of body weight per day) but accumulated fat at a rate of 5.4±0.3 g per kilogram per day — about three times that in the fetus, as confirmed by increased skin-fold thickness. How this change in body composition affects the future growth of formula-fed premature infants, and how body composition is altered by other dietary regimens such as the provision of human milk, remain to be determined. (N Engl J Med. 1981; 305:1495–500.) The Committee on Nutrition of the American Academy of Pediatrics suggests that the “optimal diet for the low-birth-weight infant may be defined as one that supports a rate of growth approximating that of the third trimester of intrauterine life.” 1 This rate of growth is commonly equated with the rate of weight gain, without reference to the ingredients of this gain and consequent body composition. Since there is currently no better clinical criterion for determining the nutritional needs of premature infants, the ability to match intrauterine growth rates “remains a widely used target of achievement.” 2 Special formulas have been designed to.