Acute encephalitis: etiology, clinical findings and prognosis. We studied 44 patients with acute encephalitis diagnosed in a neurological university clinic during an 11-year period. An etiology was found in 11 cases (25%). In 3 the cause was herpes simplex virus; in 2 morbilli. There were single patients in which the cause was mycoplasma pneumoniae, epidemic parotitis, ornithosis, infectious mononucleosis, influenza B-virus and recent tetanus immunization. No specific etiology was found in 33 (75%). Besides fever the most frequent sign was impairment of consciousness in 39% of cases. Four patients (9%) died. Among the survivors mental and/or focal neurological deficits persisted in 22 (55%). Most frequent was dementia in 6 cases (15% of survivors). Impaired consciousness in the acute phase indicated a worse prognosis (p < 0.005).