The clinical picture and course and diagnostic criteria of Mollaret''s meningitis were described and differential diagnostic possibilities were discussed. Two case histories were presented to illustrate diagnostic pitfalls. The first patient, in all probability, had Mollaret''s meningitis, with onset in 1943. The second patient, mainly on the basis of cerebrospinal fluid findings, was initially diagnosed as having an "atypical Mollaret''s meningitis" and, subsequently, as having an "amebiasis of the cerebrospinal fluid." Later on, ventriculography led to the diagnosis of "congenital stenosis of the aqueduct," but critical evaluation after ten years points to the presence of a space-occupying mass in the sellar region. It is stressed that cerebrospinal fluid cytology cannot offer decisive evidence, since large mononuclear, socalled endothelial cells are also found in other conditions.