Besides its classical actions in calcium metabolism, it is clear that the hormonal form of vitamin D has many new functions, which have only been discovered as a result of following the appearance of its receptor. Furthermore, because of the vitamin D-based endocrine system, the use of vitamin D compounds in treating a variety of diseases has been expanded. We now know that 1.25-(OH)2D3 not only stimulates the intestine to absorb calcium and phosphorus, the bones to mobilize calcium and phosphorus, and the kidney to cause increased renal reabsorption of calcium, but also directly suppresses the parathyroid hormone and is a developmental hormone necessary for the recruitment of cells for osteoclast formation, for female reproduction, for development of skin, and for the treatment of certain malignant conditions.