Acute encephalopathy with biphasic seizures and late reduced diffusion (AESD) is a syndrome characterized by biphasic seizures with impaired consciousness. AESD is rare outside Asia, and consecutive cohort studies are therefore scarce. Herein, we aimed to describe the detailed characteristics of AESD, including clinical course, electroencephalogram data, laboratory data, imaging findings, treatment, and outcomes. We reviewed the clinical database and medical charts of 43 consecutive pediatric patients (<18 years old) who developed AESD between October 1, 2002, and September 30, 2019. We found that AESD occurred even though patients did not develop prolonged seizures. A comparison between the two groups (first seizure duration <30 min and first seizure duration ≥30 min) revealed three main findings: first, patients with AESD who had shorter seizures had better prognosis than those with prolonged seizures; second, patients with AESD who had shorter seizures tended to have earlier occurrence of a second seizure; and third, high signal intensity on diffusion-weighted magnetic resonance imaging was observed mainly in frontal areas, not diffusely, in patients with shorter seizures, and in a broader area in patients with prolonged seizures. Our description of the detailed clinical picture of AESD may add new insight into its pathophysiology.