Tissues from 13 exceptionally early cases of multiple sclerosis were studied to identify and characterize the primary demyelinating lesion, using a variety of histological and immunocytochemical methods. Multifactorial cluster analysis identified five significantly distinct lesion groups, which showed histological progression from simple microglial lesions, predominating in tissues from the earliest cases, to complex hypercellular fully demyelinated plaques, chiefly associated with cases of intermediate duration. Quiescent lesions showing evidence of remyelination were found at all stages of the disease studied, but hypocellular inactive plaques, were associated with older cases. Evidence is presented that initial demyelination is effected by activated resident microglia. Undegraded myelin is initially enveloped by membranes bearing fixed complexes of immunoglobulin and complement. In contrast with perivenous encephalomyelitis, in which demyelination was dominated by T-cell infiltration, multiple sclerosis lesions of comparable duration and maturity exhibited humoral immune reactions. Parenchymal CD4+ T-cell infiltration developed in association with subsequent plaque maturation. These results emphasize the need for lesion staging when multiple sclerosis tissues are being used in the investigation of pathogenic mechanisms, and suggest that further analysis of the oligoclonal B-cell response may be productive in the search for primary provoking antigens.