Four patients with pathologically documented polycystic cavitation of the brain had an acute illness characterised by stupor, seizures, CSF erythrocytic and monocytic pleocytosis, increased CSF protein, and diminished CSF glucose. The acute phase was followed by chronic decerebation, disappearance of the CSF abnormalities, and radiological evidence of polycystic cavitation of the brain. In one patient Herpes simplex was isolated from a cutaneous vesicle. The CSF abnormalities in the disorder have received scant attention, and have not previously been correlated with the acute and chronic stages. Clearly some cases are associated with Herpes simplex virus. The clinical profile should now be sufficiently distinctive to permit future identification of the aetiology in more neonates.