Down's syndrome and its association with Alzheimer's disease.

The purpose of this study was to ascertain the frequency of Alzheimer's dementia in the relatives of Down's probands and to establish whether these two disorders cluster together. Out of 128 families of Down's probands, only 67 responded to the questionnaire. Twelve cases of Alzheimer's dementia were found in the eight families of 67 probands. There was a significant increase in the frequency of presenile dementia, but no significant increase in senile dementia among the first and second degree relatives of these probands. The methodological problems were discussed and future directions suggested.