In the sunnier parts of the world a dietary intake of vitamin D is unnecessary,* but for most of us it is a true vitamin. Its dihydroxylated metabolite 1,25-dihydroxyvitamin D3 (l,25(OH)2D3) is, however, a hormone synthesised in the kidney and borne by the bloodstream to distant target organs (gut, bone, and possibly the parathyroid glands) with a complex feedback mechanism regulating its rate of production.1 None of the other vitamin D metabolites has yet been confidently assigned a hormonal role in man. The unravelling of the interrelationships between vitamin D and its metabolites is one of the triumphs of recent medical research, and the stage has now been reached when treatment with synthetic metabolites of vitamin D is beginning to be available for several disorders. The major functional categories of endocrine disease are all represented among defects in the metabolism of l,25(OH)2D3. Hormone deficiency may occur from failure of tubular cell function; impairment of feedback control may result in subnormal or excessive production; or the disorder may be due to unresponsiveness in the target organs. Physiological considerations?Cholecalciferol (D3) is formed in the skin under the influence of ultraviolet light.2 Its pre cursors remain in situ because they are insoluble in water and