We will conduct a randomized open-label controlled trial of oral cholecalciferol supplementation in children aged 1-21 years with stage 2-5 (pre-dialysis) CKD receiving regular nephrology follow-up at a tertiary-care children's hospital (Johns Hopkins Children's Center). The intervention model will be parallel assignment. Children will be randomly allocated to receive either cholecalciferol supplementation 4000 IU per day (28,000 IU weekly) according to the KDOQI recommended supplementation for children with mild 25D deficiency, or will be treated with 400 IU/day (2,800 IU/weekly), which is the recommended dietary allowance.(1) Allocation will be double-blinded to prevent knowledge of allocation status affecting interpretation of results. Study participants will be prescribed any clinically indicated additional cholecalciferol supplementation once the 3-month laboratory measures have been obtained, based on serum 25D levels at the end of the study period.(1) We are proposing a supplementation dose of 4000 IU/day in children with CKD, which is considered a safe dose by KDOQI standards; vitamin D2 doses as high as 10,000 IU/day have been given to French patients with CKD for > 1 year with no evidence of vitamin D toxicity.(2) 25D levels between 40 and 80 are indicative of "sufficiency", and a safe upper limit of intake, in which the risk of hypercalcemia is negligible, has been defined as 10,000 IU/day.(3, 4) Cholecalciferol will be provided in both tablet (vitamin D 2000 IU tablets and 400 IU tablets, National Vitamin Company, Casa Grande, AZ) and liquid (Enfamil® D-Vi-Sol™ Drops, 400 IU per mL) form, based on the ability to tolerate and preference of the subject.