We evaluated the presence of soluble Fas molecule (sFas) in the cerebrospinal fluids (CSF) and the sera of patients with multiple sclerosis (MS) or human T-lymphotropic virus type I (HTLV-I) associated myelopathy (HAM) using an enzyme-linked immunosorbent assay (ELISA). Patients with multiple sclerosis in the active phase had higher sFas serum levels than control (p < 0.005). In addition, significantly increased serum levels of sFas were found in patients with HAM (p < 0.005). We found a significantly increased CSF levels of sFas in patients with HAM and patients with MS in the active stage (p < 0.005). These results suggest that serum sFas may be related to clinical activity in patients with MS and that Fas may play an important role in the pathogenesis of HAM.