Background and Significance Vitamin D is a fat-soluble vitamin necessary to maintain bone health as it maximizes calcium absorption from the gastrointestinal tract. Our primary source of vitamin D is through skin synthesis following exposure to solar UV-B irradiation. Natural dietary sources are limited which has led to the fortification of milk and baby formula with vitamin D.1 Vitamin D deficiency reduces dietary calcium absorption to 10-15% whereas 30 to 40% is absorbed when vitamin D levels are adequate.2 Although the effect of vitamin D is primarily on calcium homeostasis, vitamin D receptors can be found on many different cells within the body suggesting vitamin D has an impact beyond bones. Several epidemiological studies have shown an association between vitamin D and the prevalence of multiple sclerosis3, type 1 diabetes4;5, and cancer6. More recently, low vitamin D has been associated with lower lung function7;8 raising the possibility that vitamin D may influence pulmonary status. The active metabolite, 1,25 dihydroxyvitamin D, has a number of actions that may be relevant to respiratory diseases. It inhibits the formation of matrix metalloproteinases, inhibits fibroblast proliferations and influences collagen synthesis, all of which could influence tissue remodelling in the pulmonary airways.