Acute herpes simplex encephalitis (HSE) has a grave outcome, and detection of prognostic features is of clinical importance. Thirty patients with HSE were assessed in a retrospective study. Diagnosis was confirmed by serological methods using the indirect immunofluorescence technique (IFT). Antiviral treatment was given to 23 of the patients. Focal convulsions were more frequent in patients below 18 years of age, while confusion and memory disturbances were prevalent among patients above 18. The final outcome was influenced by the degree of severity of the disease at the peak and the state of consciousness and duration of disease prior to the initiation of anti-viral treatment. No correlation was found between antibody levels in serum or in CSF and the outcome. We conclude that the clinical degree of severity the duration of illness prior to treatment and state of consciousness at the initiation of anti-viral treatment are of prognostic importance.