Vitamin D and PTH are major hormones that regulate calcium metabolism. Also, the serum level of phosphorus is known to change according to the level of calcium and to regulate the vitamin D and PTH. Recently, cloning of 1alpha- hydroxylase cDNA which is the key enzyme in vitamin D metabolism, enabled us to examine the effects of phosphorus in vitamin D metabolism at a molecular level. Furthermore, the direct effect of phosphorus in PTH synthesis is being elucidated in recent reports. In this paper, we summarized the regulation of vitamin D and PTH by phosphorus.