BACKGROUND: Specific sub-groups of the population, such as older people, are at increased risk of poor vitamin D status. There is growing evidence, however, that vitamin D insufficiency may be a more widespread problem. AIM: To determine the prevalence of vitamin D insufficiency in a group of healthy Irish adults. METHODS: Serum 25-hydroxyvitamin D was measured by radioimmunoassay in 70 healthy Irish adults working in South Dublin. RESULTS: Overall, half of the group (51%) had vitamin D insufficiency; this was more pronounced in wintertime, with prevalence rising to 58%. Winter was a significant negative predictor of serum 25-OH D levels. Sixteen percent of the participants reported taking a vitamin D-containing supplement (multivitamin), but this did not significantly influence the serum 25-OH D levels. CONCLUSION: Vitamin D insufficiency is not confined to "at-risk groups" and is likely to be prevalent in the general public in Ireland, especially in wintertime.