Encephalitic illnesses with Epstein-Barr virus (EBV) only represent a small percentage of the causes of viral encephalitis. Clinical symptoms are not specific. Biological standards tests carry few elements of direction, only serologic test of infectious mononucleosis confirms the diagnostic of recent EBV infection. Previous observation shows that the initial study can present similarities with herpes simplex encephalitis, which forces the immediate start of treatment by Acyclovir.