Herpes simplex encephalitis (HSE) is one of the fulminant necrotizing, often fatal sporadic form of the encephalitis caused by herpes simplex type I virus. Characteristically, there is early and almost constant involvement of one or both temporal lobes, although there are common additional areas of involvement. Appropriate early treatment following correct diagnosis by clinical findings, CSF study, EEG and several radiological studies including angiography, redionuclide studies, CT or MRI can reduce its mortality and severity of the sequelae. We report a case of HSE diagnosed by adjuvant study of $^{99m}Tc-HMPAO$ regional cerebral blood flow SPECT, which showed a marked increase in bitemporal cerebral blood flow in a 24-year-old man.