The absorption and retention of calcium and phosphorus in infants of 28 weeks' gestation, or less, who were fed either breast milk or a preterm formula, were determined in 22 balance studies. Breast milk contained significantly lower amounts of calcium and phosphorus than the formula milk. The percentage absorption of calcium was similar in the two groups, but the infants fed breast milk had greater urinary calcium losses and significantly lower calcium retention than the group fed formula milk. The percentage phosphate absorption was also similar in the two groups but because of the higher phosphate intake in the formula milk this group retained a significantly greater amount of phosphate. The extremely low birthweight infants fed on breast milk were phosphorus depleted, with low plasma phosphate concentrations, and seemed to retain as much phosphate as possible. The hypophosphataemia may result in reduced deposition of calcium in bone and explain the calciuria in the breast fed infants. Substrate deficiency may be an important factor in the aetiology of rickets of prematurity and, particularly if breast milk is used, preterm infants may require calcium and phosphate supplements from an early age.