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The presenile dementia which almost invariably complicates Down's syndrome after the fourth decade is histologically identical to Alzheimer's presenile dementia. Ultrastructural changes in Down's syndrome dementia (DSD) have not been reported thus far. If DSD meets the rigid ultrastructural criteria established for Alzheimer's disease, then an important predictable age-dependent reservoir of material is available for multi-disciplinary studies of the earliest changes of Alzheimer's disease and aging in the human brain.Thin slices of brain tissue from a 54 years old woman with DSD were immersed in 4% phosphate-buffered paraformaldehyde (pH 7. 3) 12 hours after death, post-fixed in 1% phosphate-buffered OsO4 and embedded in Araldite. Silver impregnations of adjacent slices revealed the characteristic light microscopic findings of Alzheimer's disease: intra-neuronal neurofibrillary tangles, senile plaques, and granulovacuolar bodies.

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