In order to improve vitamin D status of children from Ushuaia (55 degrees S), at the South of Argentina, double supplementation with 100.000 IU of vitamin D was administered at the beginning of winter (March 2004), and 3 months later during winter (June 2004). In 2004, serum 25-hydroxyvitamin D (25OHD) was measured before the first supplementation, a month after, and 3 months after receiving the second supplementation (March, April and September). We studied 18 healthy children from Ushuaia, age (mean+/-S.D.) 7.3+/-4.4 years old (range 1.2-14.6), seven girls and 11 boys. Before treatment, serum 25OHD was 29.3+/-5.9 ng/ml. It increased significantly 1 month after the first supplementation (April): 35.3+/-4.4 ng/ml (p<0.001), and decreased significantly 3 months after the second supplementation: 22.4+/-4.6 ng/ml (September (p<0.001). No child was neither deficient (<10 ng/ml) nor insufficient (10-15 ng/ml) of vitamin D. On April, a month after the first supplementation, no children had vitamin D intoxication levels (>50 ng/ml). These results disclosed that to prevent vitamin D deficiency for children at zones of risk at the south of our country, double supplementation of 100,000 IU of vitamin D during autumn and winter, would be adequate and safe.