Vitamin D insufficiency is common in CF patients. Treatment of vitamin D insufficiency in CF patients requires large doses of vitamin D. Adequate vitamin D status in CF is important for skeletal health and the prevention of osteoporosis. In addition to skeletal benefits of vitamin D, recent evidence has demonstrated that vitamin D plays an important role in the regulation of the immune system by increasing anti-microbial peptides in the lung and other barrier sites. Whether improving vitamin D status in CF patients would enhance the immune system has not yet been explored in a clinical study. This would have significant clinical impact in CF care since vitamin D status remains undertreated, especially in the setting of infection. The hypothesis of this proposal is that rapid correction of vitamin D insufficiency will result in improved innate immunity by increasing production of anti-microbial peptides resulting in more effective killing of bacteria. To address our hypothesis, the following two aims are proposed: 1) To evaluate the effect of rapid correction of vitamin D insufficiency as an adjunctive therapy on production of anti-microbial peptides in acute respiratory exacerbation in CF patients 2) To determine the effect of vitamin D treatment on bacterial killing in acute respiratory exacerbation in CF patients and to correlate with free LL-37 levels in sputum. The long term objective of this proposal and of our research group is to study the role of nutrition including vitamin D to improve the immune system in the setting of infection in CF.