Fifty-two plaque or lesion areas were examined from 25 cases of multiple sclerosis. Twenty-four of these showed acute features, whereas the rest were more chronic in nature. The acute lesions showed lymphocytic infiltration (79%), fibrinous exudation (63%), lymphocytic meningitis (50%) and venulitis (58%). Of the chronic lesions, there were only 21% with lymphocytic infiltration, 11% with fibrinous exudates, none with meningitis, 29% with organising endovenulitis, 36% with fibrosed vein walls. The finding of a fibrinous inflammatory exudate in the acute lesion is a new observation in multiple sclerosis. Likewise, the observation of an inflammatory infiltrate confined to the vein wall (and often present at a distance from the plaque) has not been previously recorded in the disease. The chronic lesion, by contrast, showed relatively little fibrin, but there was considerable reparative thickening of the walls of the involved veins. The evidence provides new humoral and cellular evidence of an inflammatory process in multiple sclerosis which precedes or is not directly associated with the demyelinating process.