Refractory epilepsy, electroencephalographic abnormalities, progressive hemiplegia, and contralateral hemicerebral atrophy developed in a previously healthy 3-year, 9-month-old girl. Extensive laboratory testing showed elevated serum antinuclear antibody titers, cerebrospinal fluid oligoclonal bands, and elevated immunoglobulin G (IgG): albumin ratio, IgG index, and IgG synthesis rate: Pathological study of a subtotal hemispherectomy specimen revealed widespread cerebral vasculitis with immunofluorescence staining for IgG, IgM, IgA, C3, and C1q, and ultrastructural evidence of vascular injury in addition to severe cortical atrophy with marked neuronal loss. Cerebrospinal fluid abnormalities in other reported patients siuggest that immunological abnormalities may not be unique to this girl. These data suggest possible immunopathogenetic mechanisms in these patients.