Vitamin D insufficiency is common in patients with cystic fibrosis. The investigators study will examine a large dose of vitamin D given to patients who have cystic fibrosis and are admitted to the hospital for a pulmonary exacerbation to determine whether vitamin D can improve clinical outcomes and whether the dose given is correct. The investigators hypothesis is that vitamin D therapy will improve production of anti-microbial peptides and will increase bacterial killing of microorganisms.