MONDO_0020492 (hemimegalencephaly) is a rare cerebral malformation characterized by overgrowth of all or part of a cerebral hemisphere, often with ipsilateral severe cortical dysplasia or dysgenesis, white matter hypertrophy and dilated lateral ventricle, presenting in early infancy with progressive hemiparesis, severe psychomotor retardation and intractable seizures. Hemimegalencephaly may be an isolated finding or associated with other syndromes such as angioosteohypertrophic syndrome, epidermal nevus syndrome and Ito hypomelanosis. Management includes seizure control by antiepileptic medications and early hemispherectomy. Also known as: unilateral megalencephaly.