The brain lesions were studied by magnetic resonance imaging (MRI) in 29 HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP), 43 multiple sclerosis (MS) (11 with a primary progressive course: PPMS and 32 with a relapsing-remitting course: RRMS), 23 collagen-vasculitis and 54 HTLV-I-seronegative patients with other non-inflammatory neurological disorders (OND), according to the size of the lesions and their location in the brain (deep and subcortical white matter, periventricular white matter, infratentorial region and gray matter). Semiquantitative MRI analyses revealed the followings. (1) The frequency of either small or large lesions located in the periventricular as well as deep and subcortical white matter was significantly higher in HAM/TSP than in OND patients (p < 0.006, chi 2 test). In addition, the number of small lesions in the deep and subcortical, and periventricular white matter was significantly greater in HAM/TSP than in OND patients (p < 0.0003, Mann-Whitney U-test). (2) The HAM/TSP patients significantly exceeded the collagen-vasculitis patients in the frequency and the number of small lesions located in the periventricular region (p < 0.0001, chi 2 test, and p < 0.05, Mann-Whitney U-test, respectively), although no difference was found in case of those in the deep and subcortical white matter. (3) The HAM/TSP group had a lower frequency of small deep and subcortical white matter, small infratentorial and large periventricular lesions than the entire MS group (p < 0.05, chi 2 test). The PPMS group greatly exceeded the HAM/TSP group in the number of MRI lesions in all brain regions (p < 0.05, Mann-Whitney U-test), whereas RRMS and HAM/TSP patients showed a similar number of lesions (p > 0.1, Mann-Whitney U-test). Both the difference between HAM/TSP and collagen-vasculitis and the similarities between HAM/TSP and RRMS in the MRI features suggest that although all commonly have a vasculitic process as a necessary component, some other factor, probably with demyelinating effects, is operative HAM/TSP.