We evaluated the impact of brain MRI findings and oligoclonal IgG bands (OCBs) on conversion to clinically definite multiple sclerosis (CDMS) in 26 Japanese patients with clinically isolated syndrome (CIS). 19.2% had OCBs positivity and 3.8% had fulfillment of Barkhof criteria at baseline. 60.0% of CIS patients with positive OCBs and 9.5% of those with negative OCBs developed CDMS during 60.6months. Japanese CIS patients with positive OCBs have an equivalent risk of developing CDMS. A hypothesis that Japanese CIS patients may have substantially less OCBs positivity, MRI lesions, and conversion ratio than Caucasians, should be further tested.