Magnetic resonance imaging (MRI) of the brain was studied in 35 patients with HTLV-I-associated myelopathy (HAM)/tropical spastic paraparesis (TSP), 19 HTLV-I seropositive carriers without HAM/TSP (non-HAM/TSP carriers), 18 patients with HTLV-I seronegative spastic spinal paraparesis (SSP), and 82 HTLV-I seronegative controls with other neurological disorders. The incidence of white matter lesions was significantly higher in HAM/TSP (66%) than in the controls (23%) and SSP (11%). HAM/TSP exceeded non-HAM/TSP carriers significantly in the incidence of multiple white matter lesions (37% vs 10%). HAM/TSP affected the deep and subcortical cerebral white matter multifocally, sparing the periventricular regions. None of the lesions were enhanced by gadolinium-DTPA. HAM/TSP patients with the white matter lesions had both a longer duration of disease and a greater disability than did those without lesions. The white matter lesions gradually increased in number, as the disability status became worse, in spite of the high dose corticosteroid treatment. All these observations suggest that the MRI abnormalities of the HAM/TSP brain may reflect the chronic perivascular inflammation with progressive gliosis (chronic disseminated encephalomyelitis). We propose that brain MRI can be successfully utilized as a reliable and non-invasive measure for following the disease progression in HAM/TSP.