Autoantibodies to GluR3, an AMPA glutamate receptor subtype, may be a cause of chronic unilateral encephalitis (Rasmussen's syndrome). We report a woman with chronic left hemisphere encephalitis whose partial seizures, aphasia, and motor weakness are highly responsive to intermittent steroids and cyclophosphamide. Her serum and CSF were negative for antibodies to GluR3 by both immunoblot and immunocytochemical analysis of cells transfected with GluR3 cDNA, indicating that separate immune-mediated processes may be involved in some cases of chronic encephalitis.