A comparison has been made of the influence of feeding own mother's milk and formula on the oxidation and accretion of energy and macronutrients in the growing preterm infant of very low birth weight (less than 1,300 g) by using the combined techniques of nutrient balance and computerized indirect calorimetry. There were 22 studies in formula-fed infants and 15 studies in premature infants fed own mother's milk. Despite their lower metabolizable energy intake, the infants fed own mother's milk grew in weight, length, and head circumference at a rate approximating those of the formula-fed group. The metabolic rate was significantly lower in the infants fed own mother's milk (56.0 +/- 0.9 v 62.6 +/- 0.8 kcal/kg/d; P less than .001). The protein intake, oxidation, and accretion were similar in the two groups. The infants fed own mother's milk had a significantly lower fat intake (P less than .001), higher fat oxidation (P less than .025) and consequently lower fat accretion (P less than .001) than the formula-fed infants. The proportional fat content of the daily weight gain was lower in the infants fed own mother's milk (16% v 33%; P less than .001) but protein content was similar (13% v 12%). The accretion of energy, fat, and protein correlated with the respective metabolizable intakes in both groups (r = .81 to .98; P less than .001), suggesting that accretion rates and hence composition of weight gain are dependent on levels of energy and macronutrient intake.