In summary, there are many studies documenting vitamin D deficiency in the CF population however there is a paucity of literature evaluating treatment regimens to correct this deficiency. One may ask why not study a more clinically relevant outcome such as fracture rate or change in bone mineral density rather than a biochemical marker. Although these are definitely important clinical outcomes to examine, we feel that the CF population has unique factors that affect absorption of fats therefore it is difficult to estimate the amount of supplementation needed to achieve optimal serum levels (i.e. > 75 nmol/L) of 25OHD. Changes in serum levels can be seen within 4-8 weeks whereas changes in bone mineral density can take years to be detected. If vitamin D supplementation is inadequate to achieve optimal serum levels, changes in BMD and fractures may not be seen and the study results may be falsely negative. By determining the dosage required to achieve optimal serum levels first, this study will provide important information to researchers that can be used in long-term controlled clinical trials.