Vitamin D deficiency is endemic worldwide. The Health Council of the Netherlands advises a colecalciferol intake of 800 IU/day and an intended serum level of calcidiol of at least 50 nmol/l for people aged 50 years and older and for osteoporosis patients. In 64% of 626 fracture patients, we found a reduced serum calcidiol level according to this definition. According to calculations based on meta-analyses, a substantial proportion of fracture patients would not achieve the target calcidiol level of 50 nmol/l with a vitamin D supplement of 800 IU/day. For example, this would be the case in 37% of elderly patients with osteoporosis. Until the results of a prospective study on this are published, we propose that fracture patients are either systematically given doses of vitamin D supplements higher than 800 IU/day, or that their serum calcidiol is measured and the dose of vitamin D supplements is adjusted according to this initial value and the desired serum concentrations, and that the measurement is repeated after 3 months so that the dosage can be adjusted if necessary.