IN 1951, Leigh1 described an 8-month-old boy who showed mental retardation and motor dysfunction, as an instance of a new degenerative disease of the central nervous system. Characteristic pathological findings were bilateral symmetrical necrosis of the central gray matter, mainly in the brain stem, with prominent vascularity. A report in 1954 by Feigin and Wolf2 described three patients in two different families, emphasized the histopathological similarity to Wernicke's encephalopathy in adults, and pointed out the possibility of an inborn error of thiamine metabolism. In 1957, Richter3 reported similar pathological findings in three new patients, concluding that his cases, together with those of the two previous reports, constituted a distinctive entity. Since then case reports have increasingly accumulated,4 -22 although the disease is as yet not widely known. As proposed by Leigh, the disease has been commonly called infantile subacute necrotizing encephalopathy (more accurately, encephalomyelopathy).