Sixty patients with acute viral encephalitis admitted to The London Hospital in the last fifteen years have been reviewed. These consisted of 12 patients with known viral infection, 29 patients with acute viral infection of undetermined type, and 19 patients in whom an encephalitic illness followed a viral infection (post-infection encephalitis). The patients with primary viral encephalitis presented with an inflammatory brain disorder, including headaches and fever, and developed focal or diffuse neurological signs. Patients with post-infection encephalitis, usually following a 'flu'-like illness, presented with an acute neurological disturbance. The results of investigations, including virological studies, CSF examination, electroencephalography and neuroradiology, are described. Biopsy or autopsy material was available in 11 patients and these pathological findings supported the clinical classification of these patients. The mortality was highest in patients with herpes simplex virus encephalitis and lowest in patients with post-infection encephalitis. A considerable morbidity was found, not only in patients with primary viral encephalitis, but also in patients with encephalitis of unknown aetiology and post-infection encephalitis.