The detailed clinical time course in acute disseminated encephalomyelitis (ADEM) from initial symptoms, through exacerbation, to remission has not been widely reported. Hence, this study aimed to investigate the clinical time course of pediatric ADEM. This was a multicenter retrospective study based on registry data from medical chart reviews. The study included children who met the international consensus diagnostic criteria for ADEM. The patients comprised 18 boys and 6 girls, with a mean age of 5.5 ± 3.3 years at onset. From onset, the time until peak neurological symptoms, time until initial improvement, and time until full recovery was 3.1 ± 3.7 days, 6.0 ± 4.5 days, and 26 ± 34 days, respectively. Twenty-three (96%) patients were treated with high-dose methylprednisolone (mPSL) with a mean duration of 4.1 ± 4.0 days from onset. The condition of 15 patients (65%) improved within 3 days of high-dose mPSL initiation, whereas, that of four patients began to improve after >5 days of high-dose mPSL initiation. Only one patient (4%) did not achieve full recovery despite treatment with high-dose mPSL, intravenous immunoglobulin, and plasma exchange. This study presents the detailed clinical time course in pediatric ADEM in Japan. Progression of neurologic deficits typically lasts a few days, with initial improvement in 1 week leading to full recovery within 1 month.