1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] inhibits production of IL-12, a cytokine involved in the development of Th1 cells and in the pathogenesis of Th1-mediated autoimmune diseases. Here, we show that 1,25(OH)(2)D(3) and a non-hypercalcemic analogue are selective and potent inhibitors of Th1 development in vitro and in vivo without inducing a deviation to the Th2 phenotype. Administration of 1,25(OH)(2)D(3) or its analogue prevents chronic-relapsing experimental allergic encephalomyelitis (CR-EAE) induced by the myelin oligodendrocyte glycoprotein (MOG) peptide 35 - 55 (MOG(35 - 55)) in Biozzi AB / H mice. The inhibition of EAE induction is associated with a profound reduction of MOG(35 - 55)-specific proliferation and Th1 cell development. Importantly, the non-hypercalcemic analogue also provides long-term protection from EAE relapses induced by immunization with spinal cord homogenate when administered for a short time at symptom onset or even after the first peak of disease. Neuropathological analysis shows a reduction of inflammatory infiltrates, demyelinated areas and axonal loss in brains and spinal cords of treated mice. These resuls indicate that inhibition of IL-12-dependent Th1 cell development is associated with effective treatment of CR-EAE and suggest the feasibility of an approach based on low molecular weight inhibitors of IL-12 production in the treatment of multiple sclerosis.