Vitamin D plays a crucial role for the prevention and treatment of both postmenopausal and senile osteoporosis and possibly for a number of other conditions. The prevalence of vitamin D deficiency remains extremely high among elderly subjects. This indicates that both the perception of the problem and the interventions so far implemented remain inadequate. This is further complicated by the observation that the lower normal range of 25-hydroxycholecalciferol (calcidiol) and the minimum daily allowance of vitamin D have been recently set to values considerably higher. The association of calcidiol with the specific osteoporosis treatment, for example associating the two therapies on one specific day (once weekly) is an efficient way for increasing compliance and effectiveness of the osteoporosis therapy. The weekly dose of calcidiol may be adapted to individual needs, i.e 4,000 to 6,000 IU (or 20-30 drops) according to age or risk factors for vitamin D deficiency.