BACKGROUND: Acute encephalopathy with biphasic seizures and late reduced diffusion (AESD) is clinically characterized by biphasic seizures associated with mild to severe neurological sequelae and is the most common subtype of acute encephalopathy in Japan, accounting for around 30 % of cases. The present study retrospectively analyzed the utility of electroencephalography (EEG) in determining the optimal method of diagnosing AESD at the early stage. METHODS: This study explores early power value differences to differentiate acute encephalopathy from prolonged febrile seizure (FS). The subjects were patients aged six months to four years who had received intensive care for febrile status epilepticus and had a continuous EEG record up to 24 h after seizure onset. Power spectrum analysis of consecutive, 30-minute recordings at two-hour intervals were conducted, and the power values (alpha, beta, delta, theta waves) were compared between AESD group and FS group. RESULTS: Eight patients with AESD and 69 patients with FS were identified retrospectively. The emergence of delta waves in the frontal region was significantly higher in the AESD group at six to ten hours after onset. The emergence of beta waves across all the regions was lower in the AESD group from six hours after seizure onset. CONCLUSIONS: Frequency analysis of EEG in the early period after febrile status epilepticus onset demonstrated a significant difference between the AESD and FS groups. Delta wave power values in the frontal region at six to ten hours after onset might be useful for the early differentiation of AESD from FS.