A 55-year-old man with a subacute onset of slurred speech, ataxia, nystagmus, extrapyramidal rigidity, decreased tendon reflexes, vomiting, bilateral optic atrophy, and clonic jerks died of bronchopneumonia and respiratory failure. Neuropathological examination showed lesions characteristic of subacute necrotizing encephalopathy. Clinicopathological observations of reported cases of Leigh's syndrome in the adult are reviewed.