We report a 24-month-old male who developed rapidly progressive subacute sclerosing panencephalitis 17 months after measles infection. This patient had a history of measles infection at the age of 7 months and manifested acute encephalitis 1 month later. Developmental delay observed after encephalitis began to improve after a few months. His control electroencephalogram was normal at the age of 14 months. He was admitted to the hospital with flexor spasms and sudden head drops at the age of 24 months. His electroencephalogram revealed slow waves in the posterior regions of the brain. Vigabatrin was begun; his seizures increased with vigabatrin, and adrenocorticotropic hormone was added to the therapy. After five doses of adrenocorticotropic hormone, his clinical findings deteriorated rapidly. His second electroencephalogram revealed periodic discharges synchronized with myoclonias. He was diagnosed as having subacute sclerosing panencephalitis on the basis of clinical and laboratory findings. He lapsed into a vegetative state within a week and died at the age of 25 months. We report this rapidly progressive case to emphasize the importance of recognition of subacute sclerosing panencephalitis before applying steroids in children with myoclonic seizures. (C) 2004 by Elsevier Inc. All rights reserved.