During the past 4 years we have had on long term parenteral nutrition 70 infants. In 7 of these patients an important hypophosphatemia (Pi below 20 mg/L), associated to neurological symptoms in one case, was observed without any significant change of serum Ca concentration. The purpose of the present study was : 1) To analyze the factor(s) responsible of phosphorus depletion during long-term parenteral nutrition. 2) To determine the amounts of I.V. Calcium, phosphorus and nitrogen necessary for the prevention of such accidents. This investigation was done by stable balance technique under a constant caloric intake (100 Cal/Kg/24 h); calcium, phosphorus and nitrogen intakes were varied simultaneously or alternatively : Ca from 25 to 60 mg/Kg/24 h, P 10 to 60 mg/Kg/ 24 h, and N 320 to 640 mg/Kg/24 h. The subjects studied included 9 infants. Results are as follow : 1) Phosphorus balance is highly correlated to calcium, nitrogen and caloric intakes; 2) When excessive amounts of protein and/or calcium are given intravenously, a state of phosphorus depletion, most probably secondary to cellular anabolism, may occur; and 3) A caloric intake of 100 Cal/Kg/24 h, a calcium intake of 35 mg/Kg/24 h, a nitrogen intake of 400 mg/kg/24 h and a phosphorus intake of 40 mg/Kg/24 h will prevent this accident and promote a satisfactory development of the child.