Four patients are described with an encephalitic illness identical to that described by von Economo. Electroencephalographic, evoked potential and autopsy data suggest that involvement of the cerebral cortex is more extensive than has been generally recognized. Serological tests and viral cultures failed to reveal the infectious agent but the presence of oligoclonal IgG banding in the cerebrospinal fluid in 3 of the patients during the acute phase of the illness would be in keeping with a viral aetiology.