A 5-year-old female presented with prolonged afebrile right-sided focal seizures, right brachio-facial paralysis, and dysarthria; consciousness was not altered. Fever appeared 20 hours after onset of neurological symptoms. At admission (day 1) cerebral computerized tomography and cerebrospinal fluid (CSF) analyses were normal including undetectable alpha-interferon (alpha-IFN) and negative herpes simplex virus (HSV) polymerase chain reaction (PCR). Acyclovir was started at a dosage of 60mg/kg/day for 21 days and neurological symptoms improved. Cerebral magnetic resonance imaging (MRI) showed lesions in the left thalamus and left parietal lobe. On day 8, CSF contained an elevated leukocyte count with a predominance of lymphocytes, but alpha-IFN and HSV DNA were still undetectable. Delayed intrathecal synthesis of specific anti-HSV antibodies was found on day 26 and confirmed herpes simplex encephalitis (HSE) diagnosis. Twenty months after this episode, the patient presented with a febrile meningeal syndrome. PCR detected HSV DNA in CSF and cerebral imaging showed a new left temporal lesion. At relapse onset, intrathecal synthesis of specific anti-HSV antibodies had disappeared. Acyclovir was started at a dosage of 60mg/kg/day for 21 days and neurological status improved. At discharge, neurological examination showed right hemiparesis and bucco-facial dyspraxia. Diagnostic problems of HSE diagnosis in children are highlighted. It is suggested that the premature disappearance of intrathecal synthesis of a specific anti-HSV antibody might play a permissive role in the resurgence of cerebral viral replication.