Multiple sclerosis (MS) in Asian populations is characterised by the selective and severe involvement of the optic nerve and spinal cord as well as low prevalence rates. 15-40% of cases of MS in Japan are of this "opticospinal" type. This form of MS generally has a higher age at onset and a higher female to male ratio than conventional MS. Opticospinal MS is also characterised by frequent relapses, severe disability, few brain lesions visible on MRI, long lesions extending over many vertebral segments visible on spinal-cord MRI, pleocytosis and an absence of oligoclonal bands in the CSF, and a pronounced shift in the responses of T-helper-1 and T-cytotoxic-1 cells throughout relapse and remission phases. Conventional MS in Japanese people is, like MS in white people, associated with HLA-DRB1*1501, whereas opticospinal MS is associated with HLA-DPB1*0501. In Japanese people born after modernisation in the 1960s, the ratio of conventional to opticospinal MS has increased rapidly. Opticospinal MS is likely to have a distinct immune-mediated mechanism, which is not operative in conventional MS.