All the human herpesviruses may cause central nervous system (CNS) diseases, including benign aseptic meningitis or fatal encephalitis. It has recently been stated that human herpesvirus 6 (HHV-6) may also be neuropathogenic in children after primary infection, while in the adult, cases of fatal encephalitis have been reported only in immune-compromised hosts such as AIDS patients, and in one case of an immunosuppressed bone marrow transplant patient. We describe a multiple sclerosis (MS) patient, carrier of HHV-6 latent infection, who experienced an acute inflammation of the CNS diagnosed as encephalitis. HHV-6 specific genomic sequences have been detected by PCR in the patient's PBMCs DNA collected before and during the encephalitis. The PCR performed in the CSF in course of the acute episode was positive, while the CSF collected before the encephalitis was negative. This finding is consistent with an acute encephalopathy caused by the reactivation of a HHV-6 latent infection within the CNS, in a patient with altered immune response due to MS.