Investigations of a patient with osteogenesis imperfecta revealed deranged primary and secondary phosphorus metabolism. Absorption and retention of phosphorus were below normal. The blood-serum phosphorus was low and the product of serum calcium and phosphorus values was below normal.

The tooth was defective in development, particularly the dentin and the cementum, both of mesodermal origin. The enamel, of ectodermal origin, was softer than normal and the "brown striae" and the "lines of Schreger" were absent.

The gastric secretion contained no free hydrochloric acid and low total-acid values. This was probably a factor in the poor absorption of phosphorus.

The mother of the patient was found to have hyperthyroidism which was probably an etiological factor in the abnormal bone development and in the abnormal gastric secretion of the child.

The administration of dilute hydrochloric acid and a high-protein diet, to provide phosphorus, and cod-liver oil, for its effect on secondary metabolism of calcium and phosphorus, resulted in better digestion of food, a normal product of serum, calcium, and phosphorus values, and clinical and roentgenographic evidences of improvement.