Seventy-eight pediatric subjects with SLE and 25(OH)D levels ≤ 20 ng/mL will be randomized in a 1:1 ratio to receive either standard-dose (400 IU/day) or high-dose (6,000 IU/day) vitamin D3 for 18 weeks based upon weight at baseline. Subjects randomized to the high-dose vitamin D3 treatment arm will receive 6,000 IU per day from baseline until the subject's vitamin D levels reach ≥ 40 ng/mL at which point the vitamin D3 dose will be reduced to 4,000 IU per day. Subjects randomized to the high-dose treatment arm weighing < 40 kg will receive supplementation five days per week and all other subjects will receive supplementation seven days a week.