Acute encephalopathy in childhood is a wide and heterogenous neurological category, composed of several syndromes or diseases yet to be fully established. Recent progresses in diagnostic radiology and laboratory tests has allowed us to elucidate the nature of this huge entity. Acute encephalopathy with febrile convulsive status epilepticus (AEFCSE) is the most common type of acute encephalopathy in childhood in Japan, which develops with prolonged febrile convulsion, followed by mild unconsciousness, then subsequently provoking a cluster of convulsions (late seizures) with comatous state. Diffusion-weighted MR images shows high signal in the subcortical white matter as with cerebral lobar distribution pattern. "Acute infantile encephalopathy predominantly affecting the frontal lobes (AIEF)" is the most common subtype of AEFCSE, which is unique in that frontal lobes are the primary focus for the neurological manifestations and radiological features.