Myelin oligodendrocyte glycoprotein (MOG) is a key CNS-specific autoantigen for primary demyelination in multiple sclerosis. Although the disease-inducing role of MOG has been established, its precise function in the CNS remains obscure. To gain new insights into the physiological and immunopathological role of MOG, we determined the 1.8-Å crystal structure of the MOG extracellular domain (MOG_ED). MOG_ED adopts a classical Ig (Ig variable domain) fold that was observed to form an antiparallel head-to-tail dimer. A dimeric form of native MOG was observed, and MOG_ED was also shown to dimerize in solution, consistent with the view of MOG acting as a homophilic adhesion receptor. The MOG_35-55 peptide, a major encephalitogenic determinant recognized by both T cells and demyelinating autoantibodies, is partly occluded within the dimer interface. The structure of this key autoantigen suggests a relationship between the dimeric form of MOG within the myelin sheath and a breakdown of immunological tolerance to MOG that is observed in multiple sclerosis.