The value of a brain biopsy in diagnosis and management of suspected herpes simplex encephalitis was studied in 29 patients (16 prospectively and 13 retrospectively). The biopsy showed herpes simplex encephalitis in eight, culture-negative encephalitis in 14, and was normal in three patients. It provided an alternative diagnosis in four patients, for two of whom curative treatment was available. The biopsy was complicated by a fatal intracranial haemorrhage in one patient. The low yield of alternative diagnoses suggests that a brain biopsy is not justified in the routine investigation of focal encephalitis.