By photon absorptiometry, extrauterine bone mineralization was evaluated in preterm infants (less than 1,600 g birth weight) fed either a commercial premature formula containing 117 mg calcium, 58.5 mg phosphorus/100 kcal, the same formula containing higher phosphorus (82 mg/100 kcal), the same formula with higher calcium (140 mg Ca) and phosphorus (82 mg/100 kcal), or their own mother's milk. All infants had serum protein, albumin, calcium, phosphorus, bicarbonate, 25-hydroxyvitamin D, and alkaline phosphatase levels done at the start of the study and every 2 weeks until they weighed 1,900 g. At the start of the study, birth weight and gestational ages were similar in all four groups. There were no biochemical differences among the four groups except for a lower serum P in the human milk group. The human milk group had lower bone mineralization rate compared with the three formula groups. Bone mineral content was similar in the three formula-fed groups. However, only formulas containing 117 mg Ca and 58.5 mg P or 140 mg Ca and 82 P mg/100 kcal approximated intrauterine bone mineralization. Human milk fed infants did not approximate and were significantly different from the intrauterine rate.