Several neurological or psychiatric disorders are caused by common viruses. Herpes simplex virus type 1 (HSV1) has been proposed as a possible etiologic pathway in Dementia, Depressive Disorder, Psychosis and Schizophrenia. Encephalitis is a rare but serious complication of herpes viruses. The symptoms associated with HSV are thought to be variable, although it often begins with a clearly defined constellation of symptoms of sudden onset such as headache, vomiting, fever, and altered sensorium. In this paper, the phenomenon of dementia that develops after the herpes encephalitis will be discussed. The 49 year-old male patient was brought to the emergency service due to the clouding of consciousness, high fever and disorganized behavior. In LP realized to the patient, in BOS, 300 cells were observed; it was determined that the protein increased in the BOS biochemistry. Acyclovir 3*750 mg, seftriakson 2*2 gr treatment was started for the patient. Upon the observance of the epileptic discharges in the EEG taken, the carbamazepine 800mg/day was started. In MR imaging of the patient, the appearance compliant with the herpes encephalitis was determined.(Involvement areas in the bilateral temporal lobe front sections, bilateral amygdala, hippocampal and hemispheric complexes) In mental state examination, there were insufficiency in the person, time and place orientation, purposeless behaviors, sleep disorder, hyperactivity, disorganized speech being not appropriate to the question, insufficiency in the behavior planning and disorganized-inappropriate behaviors. Haloperidol 5mg 2*1 was started for the patient; in the follow-ups due to the EPS development, it was decreased as 2*0.5. Due to the fact that the patient’s EPS sensitivity depending on the squeal in CNS was high, quetiapine 400 mg risperidon 2 mg was started; even though there were decreases in his mobility in his follow-ups, the weakness in the cooperation and orientation was continuing. Herpes simplex virus encephalitis (HSVE)) typically affect anterior temporal lobe structures. On reactivation of HSV1, the particular apoE variant of the host might affect the number of cells infected by the virus, or the number of viruses released—thus determining the extent of damage. As a result, an increasing number of patients now recover from the disease but are left with neurological squeal ranging from severe dementia with markedly disturbed behavior to mild impairment of memory and speech. In acute Herpes encephalitis, which persists latently thereafter in the peripheral nervous system- usually in the trigeminal Ganglia and after viral activation occurs acute necrotizing hemorrhagic involvement in temporal and frontal lobes in CNS.