Platelet endothelial cell adhesion molecule, PECAM-1 (CD31) is a 130-kDa glycoprotein, member of the immunoglobulin superfamily, which is involved in transendothelial migration of leukocytes. It is expressed on endothelial cells, lymphocytes, monocytes, neutrophils, basophils and platelets. We present data showing that soluble PECAM-1 is significantly increased in sera of multiple sclerosis patients with active, gadolinium enhancing lesions. Since we have found increased levels of sPECAM-1 in sera of patients with active MRI lesions, but not in those of patients without enhancing lesions, the molecule may be regarded as marker of MS activity. Soluble PECAM-1 may be involved in immunomodulatory mechanism leading to inhibition of migration of leukocytes in MS.