Oligoclonal IgM bands (OCMB) against myelin lipids predict an aggressive multiple sclerosis (MS) course. However, the clinical significance of OCMB without lipid specificity, present in other MS patients, remains unknown. We describe here a characterization of these antibodies and study their role in MS progression. Fifty-four MS patients showing CSF-restricted OCMB were included in this study at disease onset and followed-up during 61.1 +/- 2.7 months. The specificity of OCMB and the CSF B-cell profile were investigated. A second CSF IgM study was performed in a group of eight patients. Thirty-eight patients showed OCMB against myelin lipids (M+L+) and other sixteen had OCMB lacking this specificity (M+L-). The CD5+ B cell subpopulation, responsible for most persistent IgM responses, was considerably higher in M+L+ than in M+L- patients (3.3 +/- 0.6% versus 0.8 +/- 0.2, P = 0.009). In addition, M+L+ bands persisted during disease course, while M+L- disappeared during follow-up. M+L+ patients suffered more relapses (4.2 +/- 0.6 versus 1.6 +/- 0.3, P = 0.002) and reached higher disability (EDSS score of 2.2 +/- 0.2 versus 1.2 +/- 0.2, P = 0.02) than M+L- group. These data corroborate that anti-lipid OCMB associate with an aggressive MS course and show that OCMB that do not recognize myelin lipids represent a transient immune response related to a more benign disease course.