Rasmussen's syndrome is a devastating epileptic disorder of young subjects characterized by pharmacoresistent partial sensorimotor convulsions, hemiparesis, and intellectual decline. The morphologic correlate of Rasmussen's syndrome is a chronic focal inflammation of the cerebral cortex leading to unilateral brain atrophy. The authors review light microscopic findings of three cases, where material was obtained by corticectomies of various extent. Patients included two young females, 6 and 8 years of age and a 21-year-old man with characteristic clinical features. Histology showed focal cortical infiltrates of lymphocytes, microglial nodules and neuronal loss with reactive gliosis. Immunohistochemically, lymphocytes were shown to be of T-cell lineage, while activated microglial cells were identified upon their expression of MHC II antigens. Prominent perivascular astrocytic reaction, as evidenced by the pattern of GFAP-staining, suggests recurrent injury to the blood-brain-barrier. The presence of Cytomegalovirus and Herpes simplex virus I and II could not be detected. The etiology of Rasmussen's syndrome is not known. The neuropathologic alterations, however, support the role of autoimmune-mediated neuronal excitotoxicity in its pathogenesis.