Chronic obstructive pulmonary disease (COPD) impairs the movement of air in and out of the lungs and causes shortness of breath. It is usually caused by cigarette smoking, and it is also referred to as “chronic bronchitis” or “emphysema.” Patients with COPD frequently have acute worsening of their disease, which may require hospitalization. These acute events are called “exacerbations of COPD.” Patients with more severe COPD tend to have lower blood levels of vitamin D. Vitamin D treatments are currently useful for some patients to improve the health of their bones. Whether increasing the blood levels of vitamin D in patients with COPD can prevent acute exacerbations is not known.