In addition to its role in the regulation of calcium metabolism, vitamin D has a number of immunological effects. Several studies in children found an effect of oral supplementation on allergic sensitization but so far there are no population-based studies of vitamin D serum levels. We therefore examined 25(OH)-D3 status in 18 224 adults of the Third National Health and Nutrition Examination Survey. There was no effect on sensitization to a single allergen while the prevalence of allergic rhinitis increased across quartile groups of current vitamin D serum levels. The association could be due to unrecognized confounding, however, could also point towards an altered metabolism or an increased sensitivity to vitamin D in allergic patients.