Vitamin-D deficiency is no longer to be seen only as a cause of osteomalacia, rickets and osteoporosis. There is a causal relationship with muscle function and also with the functioning of our immune system. Furthermore, vitamin-D deficiency is associated with a higher risk of autoimmune diseases and several forms of malignancy, such as prostate, colon and breast cancer. Optimal serum concentration is under discussion which has already led to modification of the recommendations on vitamin-D supplementation. At present, European consensus on 25-hydroxyvitamin-D serum concentrations seems to be a minimum of 50 nmol/l and a target of 75 nmol/l. The majority of the elderly and of non-Western immigrants are deficient or severely deficient in vitamin D and correction of such a deficiency with bolus therapy should be considered. Awareness of a vitamin-D deficiency is needed in unexplained complaints of muscle weakness or muscle pain, and in the risk groups, such as the elderly, non-Western immigrants, the chronically ill, indoor living and institutionalized, those who habitually use sun protection cream, and severely obese patients.