Calcium and phosphorus form the major inorganic constituents of bone and serve a vital role in cell function and cell metabolism. Metabolism of calcium, phosphorus, and vitamin D are intimately related and the requirement of these nutrients should take into account this relationship. For high-risk infants, the use of parenteral nutrition may be associated with disturbances in calcium, phosphorus, and vitamin D metabolism. Acute alterations in calcium and phosphorus metabolism, as reflected by excessively elevated or decreased concentrations of circulating calcium and phosphorus concentrations, are frequently iatrogenic in origin and may be minimized by attention to details of fluid, electrolyte, glucose, and other non-calcium, non-phosphorus nutrients. Calcium and phosphorus content up to 15 mM each (60 mg calcium and 46.5 mg phosphorus/dl) with a calcium:phosphorus ratio of 1.3 to 1 by weight or 1 to 1 by molar ratio, at average fluid intakes (approximately 120 ml/kg/day), are suitable to maintain calcium and phosphorus homeostasis. This stable metabolic milieu is reflected by normal and stable serum concentrations of 1.25 dihydroxyvitamin D, and normal and stable renal tubular reabsorption of phosphorus. A low content of vitamin D, 25 IU/dl, in the presence of calcium and phosphorus appears to be sufficient to maintain normal vitamin D status as reflected by the maintenance of normal serum 25 hydroxyvitamin D concentrations.