The bone mineral status of healthy preterm infants fed maternal milk was compared with that of similar infants fed maternal milk with mineral supplementation. Fifty infants with birth weight less than 1600 g were fed human milk for 1 week until reaching an intake of 120 kcal/kg/d. Thereafter, infants were assigned randomly to one of three diets: (1) continued unsupplemented human milk, providing an intake of 40 to 50 mg/kg/d calcium and 23 to 30 mg/kg/d phosphorus; (2) human milk mixed with a high mineral containing formula, providing total intakes of 130 mg/kg/d calcium and 68 mg/kg/d phosphorus; or (3) human milk alone for 1 additional week, followed by human milk mixed with a powdered fortifier, providing total intakes of 160 mg/kg/d calcium and 90 mg/kg/d phosphorus. Infants fed human milk with formula supplementation, but not those fed human milk with fortifier, had significantly higher serum phosphorus concentrations and significantly lower serum alkaline phosphatase concentrations than did those fed unsupplemented human milk (P less than 0.01). Bone mineral content of the humerus, determined by photon absorptiometry, however, was similar in all three groups; values averaged 0.104 g/cm at the beginning of the study, and remained unchanged irrespective of mineral supplementation. Shortly before hospital discharge, study diets were discontinued and infants were fed standard proprietary formula or were nursed by their mothers. At 44 weeks postconceptional age (7 to 10 weeks after change in diet), infants were reexamined. Serum phosphorus concentrations increased, serum alkaline phosphatase concentrations decreased, and bone mineral content more than doubled to values comparable with those in term infants. Results at follow-up were comparable for all three initial diet groups and for infants who were formula-fed or breast-fed after hospital discharge. The lack of any significant effect of early maternal milk supplementation on bone mineralization by 44 weeks postconceptional age suggests that these methods of supplementation of maternal milk may not be warranted for healthy preterm infants.