This pilot study aims to answer the question whether monthly oral vitamin D3 supplementation, 100,000 IU, will be safe and effective in raising serum 25-hydroxyvitamin D (form of vitamin D measured in the blood) to levels considered sufficient (30 ng/mL) but well below the threshold for toxicity (150 ng/mL) in children with sickle cell disease. Information from this study will be crucial before we perform a larger clinical trial to determine the effects of vitamin D in reducing respiratory complications in patients with sickle cell disease.