The clinical course of herpes zoster associated encephalitis (HZAE) with special emphasis on the treatment with acyclovir is described from the experience in 14 own patients and 47 review cases. Immunosuppression and dissemination involved increased risk of HZAE, whereas cranial zoster implied no or only a slightly increased risk. The symptoms were mainly disturbances of mental function and ataxia. Nuchal rigidity was noted in approximately one third of cases. The median duration from dermatomal lesion to HZAE was 15 days in immunosuppressed patients versus 5 days in non-immunosuppressed patients. Abnormal spinal fluid findings included mononuclear pleocytosis, occasionally with low glucose concentration. Protein was elevated in half of the patients. Serum sodium levels were often low. Brain CAT scans were generally normal and EEGs always abnormal. Recurrence of HZAE was noted in 2 patients. Treatment with acyclovir seemed to haw a beneficial effect. The results, however, need cautious interpretation due to the hetercgenous patient material. Two patients developed signs of HZAE while on treatment with descidovir but recovered during ongoing therapy.