"Preterm" human milk fortified with protein (0.85 gm/dL), calcium (90 mg/dL), and phosphorus (45 mg/dL) was compared with unfortified preterm human milk as a feeding for low birth weight infants. Additionally, a special formula for low birth weight infants (Similac Special Care (SC), 20 cal/oz), was compared with a standard 20 cal/oz formula (Similac). Bone mineral content (BMC), as measured by photon absorptiometry, improved in the study groups fed fortified human milk and Similac SC formula during the first 6 weeks of full oral feedings. Even though the intakes of calcium in the groups fed fortified human milk and Similac SC formula approached the intrauterine requirement for Ca during the third trimester of pregnancy (150 mg/kg/d), the values for BMC in these two groups (37 to 39 mg/cm) at the completion of the study were still considerably less than the intrauterine values for radial BMC at 36 to 37 weeks gestational age (72.6 +/- 14.1 mg/cm). Furthermore, the relative phosphorus deficiency (as determined by increased urinary Ca excretion and increased renal tubular reabsorption of phosphate) in the human milk groups occurred with or without supplements of Ca and P. Rate of weight gain in the fortified human milk group was greater than that of the unfortified human milk group and was comparable to that of infants fed Similac SC formula. Rate of weight gain for the unfortified human milk group was similar to that of infants fed Similac formula containing 20 cal/oz. However, none of the four feeding groups exceeded the 50th percentile for weight at the time of discharge (36 to 37 weeks postconceptional age). The results suggest that fortifying preterm human milk with Ca, P, and protein for low birth weight infants will improve bone mineralization and rate of growth to levels comparable to those achieved with a special formula containing high amounts of protein, Ca, and P.