A male infant developed left-sided myoclonus associated with low grade fever at 21 days of age. The course of pregnancy and delivery had been uneventful. Birth weight was 2,240g at 38 weeks of gestation. Physical examination at 26 days of age revealed hyperirritability and incomplete Moro reflex. EEG showed periodic discharges originating in the right central region. Initial brain CT at 26 days of age showed no apparent abnormality. In lumbar CSF protein was 173 mg/dl, and nucleated cells 328/mm3 (polynuclear cells 6 and mononuclear cells 322). Cultures of CSF for bacteria and viruses were negative. Although, the serum titers of CF antibody to herpes simplex virus (HSV) were 1:4 to 1:8 during hospitalization and those in CSF were not increased significantly, ELISA titers for IgG and IgM in serum and CSF rose significantly. The diagnosis of HSV encephalitis was made at 41 days of age. His myoclonus was well controlled with phenobarbital and clonazepam. He was treated with acyclovir and CSF findings returned to normal. On 2nd brain CT (39 days of age), calcification at thalamus, basal ganglia and right frontal lobe, which is extremely rare CT findings for HSV encephalitis, was noticed. His DQ at discharge was about 90. It is emphasized that the early diagnosis of HSV encephalitis can be made by ELISA and serial brain CT examinations, and that HSV encephalitis should be considered when calcification of thalamus and basal ganglia are detected on brain CT.