Specific Aim 1. Determine the effect of vitamin D repletion on changes in flow mediated dilation (FMD) in vitamin D deficient SLE patients. The investigators hypothesize that 25(OH)D repletion will improve endothelial function in 25(OH)D deficient lupus patients. For this pilot study, the investigators have opted to use a Randomized Phase II screening design (36). The screening design is meant to provide preliminary comparisons of an experimental treatment to an appropriate control, with the idea that the pilot study would provide valuable information to aid in the design of a definitive Phase III evaluation, should the experimental treatment prove promising in the Phase II trial. The trial is designed to determine the effect of vitamin D repletion with D3 on FMD in vitamin D deficient SLE subjects. Approximately 50 SLE subjects will be screened for total 25(OH) vitamin D (25(OH)D) levels and inclusion/exclusion criteria. However, screening will continue only until 32 participants have been enrolled that have total serum 25(OH)vitamin D levels ≤ 20 ng/ml and meet inclusion/exclusion criteria. A baseline FMD, interferon (IFN) signature assays, and levels of circulating non- and apoptotic endothelial cells (EC) and endothelial progenitor cell (EPC) will be performed at the baseline visit. Participants will be will be randomized into two equal groups of 16 to receive one of two daily oral D3 doses previously used in supplementation trials with no evidence of harm. Group 1 (controls) will receive 400 international units (IU) of D3 daily. Group 2 will receive 5,000 IU daily. Studies of supplementation in subjects deficient in vitamin D demonstrate that supplementation with 1,000, 5,000, and 10,000 IU daily result in increases in 25(OH)D of 4.8, 36.7, and 63.8 ng/mL without evidence of toxicity (37). In this study, steady state levels were achieved at 90 days. As shown in our preliminary studies, 4,000 IU daily is safe and effective at repletion in our lupus clinic population. Some subjects had not achieved steady state at 90 days, so the invesitgators have chosen to dose for 16 weeks. The primary endpoint will be a change in FMD after 16 weeks of vitamin D repletion. The secondary endpoint will be the reduction in IFN signature and level of circulating apoptotic ECs/EPCs in response to vitamin D repletion from baseline to 16 weeks.