Human milk when fed to preterm infants is frequently supplemented with human milk fortifiers that provide an additional source of protein, energy, and minerals. Human milk that was provided by the mother of a preterm infant, and that was supplemented with commercially available human milk fortifiers, was assessed under simulated syringe-pump and bolus feeding circumstances for the delivery of energy, calcium, phosphorus, copper, magnesium, and zinc to an infant. In general, the nutrients were not completely delivered with syringe-pump feedings, with the greatest losses occurring in the concentrations of calcium and phosphorus. The losses were more pronounced with the use of a powdered fortifier than with the use of a liquid fortifier. Little or no change in the concentrations of the various nutrients were observed with simulated bolus feeding. We suggest that human milk fortified with supplements be fed with care to assure complete delivery of the nutrients and that infants receiving such feedings be monitored to assure adequate nutritional status.