Multiple sclerosis (MS) results from an aberrant, neuroantigen-specific, T cell-mediated autoimmune response. Because MS prevalence and severity decrease sharply with increasing sunlight exposure, and sunlight supports vitamin D 3 synthesis, we proposed that vitamin D 3 and 1, 25-dihydroxyvitamin D 3 (1, 25-(OH) 2 D 3) may protect against MS. In support of this hypothesis, 1, 25-(OH) 2 D 3 strongly inhibited experimental autoimmune encephalomyelitis (EAE). This inhibition required lymphocytes other than the encephalitogenic T cells. In this study, we tested the hypothesis that 1, 25-(OH) 2 D 3 might inhibit EAE through the action of IL-10-producing regulatory lymphocytes. We report that vitamin D 3 and 1, 25-(OH) 2 D 3 strongly inhibited myelin oligodendrocyte peptide (MOG 35–55)-induced EAE in C57BL/6 mice, but completely failed to inhibit EAE in mice with a disrupted IL-10 or IL-10R gene. Thus, a functional IL-10-IL-10R pathway was essential for 1, 25-(OH) 2 D 3 to inhibit EAE. The 1, 25-(OH) 2 D 3 also failed to inhibit EAE in reciprocal, mixed bone marrow chimeras constructed by transferring IL-10-deficient bone marrow into irradiated wild-type mice and vice versa. Thus, 1, 25-(OH) 2 D 3 may be enhancing an anti-inflammatory loop involving hemopoietic cell-produced IL-10 acting on brain parenchymal cells and vice versa. If this interpretation is correct, and humans have a similar bidirectional IL-10-dependent loop, then an IL-10-IL-10R pathway defect could abrogate the anti-inflammatory and neuro-protective functions of sunlight and vitamin D 3. In this way, a genetic IL-10-IL-10R pathway defect could interact with an environmental risk factor, vitamin D 3 insufficiency, to increase MS risk and severity.