Several interesting facts are observed in multiple sclerosis (MS) when it was viewed from the point of HTLV-I-associated myelopathy (HAM). The distribution of the two diseases is completely segregated each other and deeply related to races as shown in the figure. Both diseases have significant linkage to HLA; HAM being linked to HLA-DR1 with relative risk 3.8 whereas MS being linked to HLA-DR2 with relative risk 3.3. The clinical features of 363 HAM patients and 479 MS patients were compared using statistical multivariate analysis and patients with MS and HAM appeared in two distinct clusters. From the neuropathological point of view, MS is a disorder specifically related to demyelination and HAM is a disorder closely related to T-cell mediated inflammation. The biggest difference between HAM and MS is that the causative agent is known in HAM but not in MS. The studies on HAM will hopefully help to understand the pathomechanisms of MS.