The many recently published data on vitamin D have raised much interest in the medical community. One of the consequences has been a great increase in the prescription of vitamin D concentration measurements in clinical practice. It must be reminded that only the measurement of 25-hydroxyvitamin D (25(OH)D) concentration is indicated to evaluate vitamin D status. Furthermore, since vitamin D insufficiency is so common, since treatment is inexpensive and has a large safety margin, and since we already have much data suggesting that besides its classic effects on bone and mineral metabolism, vitamin D may potentially be helpful for the prevention/management of several diseases, perhaps should it be prescribed to everyone without prior testing? In our opinion, there are however groups of patients in whom estimation of vitamin D status is legitimate and may be recommended. This includes patients in whom a "reasonably" evidence-based target concentration (i.e., based on randomized clinical trials when possible) should be achieved and/or maintained such as patients with rickets/osteomalacia, osteoporosis, chronic kidney disease and kidney transplant recipients, malabsorption, primary hyperparathyroidism, granulomatous disease, and those receiving treatments potentially inducing bone loss. Other patients in whom vitamin D concentration may be measured are those with symptoms compatible with a severe vitamin D deficiency or excess persisting without explanation such as those with diffuse pain, or elderly individuals who fall, or those receiving treatments which modify vitamin D metabolism such as some anti-convulsants. Measurement of Vitamin D concentrations should also be part of any exploration of calcium/phosphorus metabolism which includes measurement of serum calcium, phosphate and PTH.