Based on the available Danish studies, vitamin D insufficiency, defined as the concentration of serum 25 hydroxy vitamin D (25(OH)D) < 50 nmol/l, is primarily prevalent among teenagers, children from immigrant families, and this especially by the end of winter. Measurement of the 25(OH)D concentration is recommended when the consequence of the test result is most likely to be different from the current advice of vitamin D prophylaxis, already advised by the Danish Health and Medicines Authority. By adhering to these current guidelines, it should be possible to prevent severe vitamin D deficiency and rickets in Denmark.