Mollaret's meningitis is a rare clinical entity consisting of recurrent attacks of meningeal irritation, which, after a sudden onset, last for a few days. The prognosis appears to be excellent, although the aetiology has not been established. In the CSF so-called endothelial cells are a typical finding, but their classification is not yet clear. In the present case immunocytological examination of CSF cells revealed that the so-called Mollaret cells are monocytes. The time course of changes in helper/suppressor ratio is similar to that in other infectious diseases of the central nervous system.