Studies of the non-inherited component of diabetes susceptibility implicate external environmental factors operating in the first year of life, suggesting the possibility to reverse the trend with the correct intervention. Recent data suggest that the vitamin D system is a potentially important target for therapeutic intervention to prevent type 1 diabetes. These data include epidemiological studies showing that vitamin D supplementation in infancy is associated with a substantially decreased subsequent risk of the disease, and animal work in the non-obese diabetes mouse model of autoimmune diabetes showing that the incidence of autoimmune diabetes increases when the animals are nutritionally deprived of vitamin D, and that the disease can be prevented using 1,25-dihydroxyvitamin D, and non-hypercalcemic vitamin D analogues. In vitro experiments suggest that the prevention seen in NOD mice may be due to combined effects of vitamin D on antigen presenting cells and activated T-cells.