Mongolians are at high risk for vitamin D deficiency because of their residence at northern latitude, reduced exposure to UV-B rays during the winter months, and a low availability of vitamin-D fortified foods. We performed a pilot study in May 2005 to estimate the prevalence of hypovitaminosis D in Mongolian school age children and to determine the feasibility of conducting a longer and larger trial with fortified milk and vitamin D supplements. In a group of 46 Mongolian children (22 girls and 24 boys) aged 9-11 years, 76% (35) had levels of 25-hydroxyvitamin D (25(OH)D) below 50 nmol/L (20 ng/mL) and 32% had levels below 37 nmol/L (15 ng/mL). After a month of consuming 710 ml of vitamin D-fortified (total 300 IU or 7.5 microg) milk daily, only 3 of the children were below 50 nmol/L (20 ng/mL) and none below 37 nmol/L (15 ng/mL). These results reveal prevalent and serious 25(OH)D deficiency among Mongolian prepubertal school age children that appears to be ameliorated by a month of consuming approximately 7.5 microg of vitamin D3 in fortified milk.