The ideal postnatal rate of bone mineralization in preterm infants is unknown. In this study the effect of a new formula on bone mineralization in very low-birth-weight infants (28 to 32 weeks' gestation) was studied using infant-adapted direct photon absorptiometry. The 81 calories/dl experimental formula (Similac Special Care) contained 50% of the carbohydrate as Polycose, 50% of the fat as medium-chain triglycerides, and a whey:casein protein ratio of 60:40. It also contained large amounts of calcium (140 mg/dl), phosphorus (75 mg/dl), and vitamin D (120 IU/dl). Over a three- to five-week period, bone mineralization increased in two-thirds of the infants and decreased in the remainder, but all values remained in the range expected for bone mineral content in utero. Serum calcium, phosphate, magnesium, alkaline phosphatase, 25-hydroxy-vitamin D, and parathyroid hormone concentrations remained stable, whereas calcitonin concentrations fell.