Encephalitis represents an important event in pediatric practice. Between the different causes Herpes Simplex Virus (HSV) seems to be the most significant etiological agent, not only for its frequency but chiefly for conditioning prognosis. The Authors report a new case of HSV encephalitis with biphasic course illustrating the most recent pathologic, diagnostic and therapeutic features in the attempt to individualize those factors, between clinical data, laboratory findings and imaging that allow early diagnosis and consequently precocious specific antiviral therapy.