Vitamin D, a hormone critical to the body's maintenance of serum calcium and phosphorus concentrations, is currently the subject of much scientific interest. Low levels of vitamin D have been observed in many populations and epidemiological studies have suggested a link between this biochemical state and a range of diseases, such as cancer, diabetes and multiple sclerosis. While the consequence of vitamin D deficiency is well documented for bone (rickets and osteomalacia), with mixed findings relating to falls and fractures, a causal link between vitamin D deficiency and these wider health outcomes has not been established. If these relationships were found to be causal, the morbidity and mortality resulting from low levels of vitamin D could be substantial; the current evidence base, however, most robustly supports the assessment of serum 25(OH)-vitamin D in the context of specific symptoms, low bone mineral density or biochemical abnormalities, rather than as an entity to treat in its own right or as the basis for a population-wide screening programme.