The past 10 years have seen a return of rickets. Clinical and/or biochemical signs of vitamin D deficiency are still found in some children and adolescents, mainly during the winter. Sunlight exposure is able to prevent vitamin D deficiency and rickets but the dramatic influence of changes in solar ultraviolet-B radiation on cutaneous vitamin D3 synthesis, related to latitude and season effects, suggest that a vitamin D supplementation may be advisable. Moreover, human milk and common foods contain low quantities of vitamin D. So, we recommend routinely 400 IU of supplementary vitamin D per day in all infants. The vitamin D requirements in low-birth-weight infants are higher than at term infants; it is recommended the use of 1000-1600 IU per day in the first months of life. Intermittent high-dose of vitamin D and vitamin D metabolites are not advisable for prophylaxis of rickets.