There is growing interest in the contribution of vitamin D deficiency to autoimmunity. It is therefore timely to review the immunologic actions of vitamin D and the evidence linking vitamin D deficiency to autoimmune disease in animal models and to systemic lupus erythematosus in epidemiologic studies. A number of recent studies have highlighted the association between systemic lupus erythematosus and vitamin D deficiency. Vitamin D deficiency skews the immunologic response towards loss of tolerance. Adding vitamin D in vitro reverses immunologic abnormalities characteristic of systemic lupus erythematosus. Multiple systemic lupus erythematosus cohorts have low vitamin D levels. The physiologic and clinical consequences of vitamin D deficiency in systemic lupus erythematosus are not entirely known. Prospective studies of vitamin D in systemic lupus erythematosus are limited, but most cross-sectional studies show an inverse relationship between levels of vitamin D and disease activity. This suggests that repletion of vitamin D may have benefits beyond bone health for patients with systemic lupus erythematosus.