Background: Multiple sclerosis (MS) is a demyelinating autoimmune disease of the central nervous system and the leading cause of lasting neurological disabilities in young adults. Increasing evidence suggests that early treatment prevents the development of disability. However, there have been no reliable serum markers to assist the early diagnosis. In addition, interferon (IFN)-, which is the major treatment for MS, is not always effective. Therefore, the development of simple serological test to help the early diagnosis and predict responsiveness to IFN- is of clinical importance. On the other hand, a transmembrane-type semaphorin, Sema4A, has been implicated in experimental autoimmune encephalomyelitis (EAE) by regulating helper T (Th) cell differentiation. Thus, we aimed to identify the implications of Sema4A in diagnosis and pathogenesis of MS. Methods: We assayed serum Sema4A in 59 patients with relapsing-remitting MS (RRMS), 22 patients with clinically isolated syndrome (CIS) and 126 patients with other neurological diseases (OND) by developing a sandwich ELISA. To identify a source of soluble Sema4A and characteristics of MS patients with high levels of Sema4A, we analyzed peripheral blood mononuclear cells (PBMCs) from MS patients and healthy controls by flow cytometry (FACS) and gene chip analysis. The effect of Sema4A was examined in vitro and in vivo using an EAE model. Findings: Sema4A was significantly increased in sera of patients with MS and CIS compared to controls. Sema4A expression was increased on the surface of DCs in MS patients and shed from these cells in a metalloproteinase-dependent manner, affecting the Th17skewing. In addition, patients with high Sema4A levels exhibited more severe disabilities, and IFN- treatment was not beneficial to those patients. Interpretation: Measuring Sema4A is a practical laboratory test to help diagnose MS and to predict responsiveness to IFN- therapy.