A case of Herpes simplex virus encephalitis (HSVE) is reported in a 5 year old boy, who relapsed twice during the acute phase of the disease. The second relapse was completely cured by increasing the daily treatment with acyclovir to a dosage of 60 mg/kg intravenously for two weeks and 30 mg/kg by mouth for a further week. The full disappearance of the symptoms and the regression of cerebral lesions could be correlated to a prompt diagnosis and to the dosage of acyclovir employed, higher than that suggested for diseases caused that was by other members of the Herpes virus family.