In a subset of multiple sclerosis (MS) patients antibodies against myelin antigens seem to be important in the demyelinating process. In this study we investigated IgM, IgA and IgG serum antibodies against the myelin oligodendrocyte glycoprotein (MOG) and the myelin basic protein (MBP) in 261 MS patients. Seventy-two per cent had anti-MOG antibodies, 59% were anti-MBP seropositive. The dominating antibody was anti-MOG IgM. A significant relationship between IgA and a progressive disease course was found. The predominance of IgGI together with the significantly associated occurrence of IgG3 against MOG corresponds to the prevailing IgGI and IgG3 isotypes in other autoimmune diseases. Patients who actually suffered from a relapse were significant more often anti-MOG and anti-MBP IgG3 seropositive than those in remission. However, patients treated either with intravenous immunoglobulins or interferon-beta showed a significant reduction of anti-MOG IgG3 antibodies.