Ten patients with human T-cell lymphotropic virus type-1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) were treated in an uncontrolled preliminary trial by oral administration of viable Lactobacillus casei strain Shirota (LcS) containing fermented milk. HTLV-1 provirus load, motor function, neurological findings, and immunological parameters were evaluated after 4 weeks. Although LcS did not change the frequencies or absolute numbers of all the examined cell surface phenotypes of peripheral blood mononuclear cells, NK cell activity was significantly increased after 4 weeks of oral administration of LcS preparation. Improvements in spasticity (modified Ashworth Scale scores) and urinary symptoms were also seen after LcS treatment. No adverse effect was observed in all the 10 patients throughout the study period. Our results indicated that LcS may be a safe and beneficial agent for the treatment of HAM/TSP; therefore randomized controlled studies are warranted.