Differentiated ("season") approach to the prophylaxis of vitamin D<sub>3</sub> insufficiency in children
synthesized by skin cells within ultraviolet light. As a result, provision with endogenous vitamin D significantly changes during the year; the maximum can be reached in summer months, lessening in winter. Straight consequence of these changes is insufficient bone mineralization in children. A correction of insufficiency of vitamin D, depending on season, seems to be well-grounded. But still there are some unsolved questions. It's not clear, when «season» prophylaxis should be started, and how long should it be prolonged. Shall we use fortified food? What criteria can be used for the control of effectiveness of these measures? There is no research on this theme in Russia. Only data from single studies allow supposing that more than a half of Russian children have vitamin D insufficiency.