We evaluated the presence of soluble intercellular adhesion molecule-1 (sICAM-1) antigen in the sera of patients with multiple sclerosis and human T-lymphotropic virus type I (HTLV-I)-associated myelopathy (HAM) using an enzyme-linked immunosorbent assay. Patients with multiple sclerosis in the active phase had higher sICAM serum levels than did control subjects (p < 0.01). In addition, a significantly increased serum level of sICAM-1 was found in patients with HAM (p < 0.001). Furthermore, we found a positive correlation with HAM sICAM-1 and tumor necrosis factor-alpha levels in the sera of patients with multiple sclerosis in the active phase (r = 0.88, p < 0.01) and in those with HAM (r = 0.86, p < 0.01). These results suggest that serum sICAM-1 may be related to clinical activity in patients with multiple sclerosis and the detection of sICAM-1 could be useful as a marker of inflammatory disease.