Fifty-one healthy prepubertal schoolchildren were followed for 13 months in a double blind study. Twenty-four of them were supplemented with 400 IU of vitamin D2 5-7 times weekly, while 27 received a placebo. The children were examined in winter both at the beginning and at the end of the study, and in the middle of the study in autumn. Mean 25-hydroxyvitamin D levels in the supplemented group were significantly higher than those in the placebo group both in autumn and in winter, when the study ended. The vitamin D supplementation did not, however, affect other vitamin D metabolites, serum calcium, albumin, inorganic phosphorus, parathyroid hormone concentrations or alkaline phosphatase activity. Moreover, the supplementation caused no alterations in the weight or height gain or bone mineral content of the distal radius of the children, and thus subclinical rickets could not be shown.