The October 2002 Case of the Month (COM). The patient was a 27-year-old woman with a history of partial complex seizures at age 7. At age 20 her seizures changed in character and became progressively worse. Neuroimaging studies showed atrophy of the right hemisphere and contralateral cerebellar atrophy. Following a biopsy, she was scheduled for a surgical procedure, but unfortunately she expired at home during her sleep a week later. Examination of the brain confirmed the hemi-atrophy of the right cerebral hemisphere and left cerebellum. Microscopic examination showed severe gliosis and perivascular lymphocytic infiltrates in many areas. A diagnosis of Rasmussen's encephalitis was made. Rasmussen's encephalitis is a chronic neurological disorder, first described in 1958. The active neurological decline lasts from 1 to 20 years and the patients then remain stable with a fixed neurological deficit and residual seizures. Pathological examination shows a chronic encephalitis confined to one hemisphere. In the active phase, neuronophagia, activated microglial cells (rod cells), microglial nodules, and perivascular lymphocytic infiltrates, are present. In the more chronic phase neuronal loss and gliosis predominate. The etiology of Rasmussen's encephalitis is unknown but viral infection and autoimmunity have been implicated. The treatment of choice is functionally complete hemispherectomy with complete disconnection of the frontal and occipital lobes.