The effect of mode of feeding on the development of rickets in very-low-birth-weight (VLBW) infants in spite of regular vitamin D supplementation was examined. Parenterally fed infants develop rickets more often than orally fed ones in spite of sufficient vitamin D supplementation. On total parenteral feeding hypophosphatemia was observed because phosphate supplementation was significantly lower than on oral feeding or in the intrauterine growing fetus of comparable gestational age. Adequate parenteral phosphate supplementation can prevent rickets.