There are two sources of vitamin D available to man: The more important source is the cholecalciferol (vitamin D3), which is produced photochemically in the skin from the provitamin, 7-dehydrocholesterol; vitamin D ingested with food is of secondary importance, but assumes a critical role when an individual is deprived of solar exposure. Vitamin D therefore is not strictly a vitamin. A deficiency of vitamin D ultimately results in osteomalacia in adults and rickets in children, and provision of sunlight or small oral doses of the vitamin can cure this bone condition. There are, however, many less common conditions in which small doses of the vitamin are ineffective, whereas larger doses of vitamin D can achieve healing of the bone disease. These conditions are collectively called vitamin D-resistant diseases and include hypoparathyroidism, genetic and acquired hypophosphataemic osteomalacias, renal osteodystrophy, vitamin D-dependent rickets, and the osteomalacia associated with liver disease and intestinal malabsorption. Unfortunately, large doses of vitamin D continue to be prescribed for a wide variety of diseases in which there is little scientific evidence of their efficacy. The benefits and dangers of high doses of vitamin D are discussed and the problems arising from inappropriate or poorly supervised treatment with vitamin D presented. The serum concentration of the active metabolite of vitamin D, 1,25 dihydroxyvitamin D is increased in certain disease states, and the pathophysiology of some these diseases are presented. The exciting developments in tumour differentiation and the role of high doses of 1,25 dihydroxyvitamin D for the control of leukaemia and other blood and skin diseases are discussed.