Perivascular cuffs in brains taken at autopsy from 6 patients with multiple sclerosis (MS) were examined by electron microscopy. Light and electron microscopy of brain revealed acute and chronic types of cuffs. The acute type of cuffs, consisting of many lymphocytes and lymphoid cells and a few lipid-laden macrophages, was usually seen in the nondemyelinated white matter or in the margin of the plaques, and infrequently within a plaque. The chronic type consisted mainly of macrophages, plasmacytoid cells, and plasma cells and was always seen within the plaques. In the parenchymal tissue of the white matter, macrophages participated in phagocytosis of myelin and axons, but no peeling or stripping of myelin sheath by inflammatory cells was observed. So-called paramyxovirus-like fuzzy filaments were observed in the nuclei of the mononuclear cells of perivascular cuffs obtained from 5 patients. The filaments were found predominantly in the acute rather than chronic type of cuffs. Specific antigens of measles and 6/94 viruses and intranuclear RNA corresponding to the filaments could not be demonstrated in the perivascular inflammatory cells by the immunofluorescence technique or acridine orange staining.