Three patients with severe motor and intellectual disabilities presented deterioration of the activities of daily living, which was revealed to be caused by prolonged non-convulsive status epilepticus (NCSE). Their condition improved by the treatment with antiepileptics. Case 1, a 4-year-old girl with profound psychomotor retardation and past history of West syndrome of unknown etiology, became unable to sit and eat orally above age of two years. EEG showed continuous generalized slow spike and wave bursts indicating NCSE. Continuous intravenous infusion of midazolam abolished EEG abnormalities of NCSE, and she regained the ability of oral feeding. Case 2, a 3-year-old boy with Angelman syndrome and past history of West syndrome, presented decreased mental response, poor oral intake and somnolence. EEG showed continuous slow spike and wave bursts, indicating NCSE. High-dose phenobarbital therapy and continuous intravenous injection of vitamin B6 were effective, and remarkably improved his psychomotor activities. Case 3, a 3-year-old boy with Lennox-Gastaut syndrome, developed decreased psychomotor activity and loss of vocalization and walking. He could not sit by himself and became nearly bed-ridden. EEG showed very frequent generalized spike and wave bursts, showing NCSE. Continuous infusion of thiopental diminished NCSE, and he could walk again. Psychomotor deterioration in patients with severe motor and intellectual disabilities may be caused by NCSE, which should not be overlooked.