The interaction of Fas ligand (FasL) with Fas-bearing cells induces apoptosis and contributes to the negative regulation of peripheral T-cell responses. Membrane-bound FasL is cleaved by a matrix metalloproteinase-like enzyme and converted to a soluble form (sFasL). Recent studies suggest that such sFasL can cause systemic tissue damage. Here we report that serum and CSF levels of soluble FasL (sFasL) are markedly higher in three active phase patients with HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). All of these patients showed higher sFasL levels in CSF than in serum. Although the HTLV-1 proviral load of patients showed no correlation with serum or with CSF sFasL, CSF sFasL levels of 14 HAM/TSP patients correlated with the anti-HTLV-1 antibody titer and neopterin concentration in CSF. These results indicate that sFasL mediated mechanisms may contribute to the inflammatory process and subsequent spinal tissue damage seen in HAM/TSP patients.