The main part of phosphorus of the human body (80 to 85 %) is located in bones. The remaining phosphorus is mainly inorga- nic phosphate. Usually, there is a relationship between calcium and phosphate in human serum. An increase of one of these components usually leads to a decrease of the other component. An elevation of serum phosphate can occur in vitamin D intoxi- cation, hypoparathyroïdism and renal failure. A decrease of serum phosphate is bound to vitamin D deficiency and hyperpa- rathyroidism.