Banked donor milk may be a reasonable substitute for mother's milk for human infants. No data on the macronutrient composition of banked donor milk have been reported. This study determined the composition of donated milk from a large number of banked donor milk samples and compared it to the reported values for macronutrients in mature breast milk. During a 9-month sampling period (May 2006 through February 2007) from a nationwide milk bank network, 415 sequential samples from 273 unique donors were analyzed for fat, protein, and lactose content, as well as energy density. Descriptive statistics were computed, including mean, standard deviation, coefficient of variation, median, and range. Percentiles were determined from the empirical distribution of the data. A ninety-five percent confidence interval was computed using standard, large sample (Gaussian) methods. Banked donor milk mean values (in weight/volume) were found to be 1.16%+/-0.25% for protein, 3.22%+/-1.00% for fat, 7.80%+/-0.88% for lactose, and mean total energy was 65+/-11 kcal/dL. Banked donor milk macronutrient content was found to differ from the values reported in the literature for mature human milk. Unformulated banked donor milk alone, similar to mother's milk alone, does not have sufficient macronutrient content or energy density to sustain a very-low-birth-weight preterm infant. Fortification could make up for these shortcomings, perhaps making formulated banked donor milk a better choice for preterm infants than bovine-based formulas when mother's milk is unavailable.