We have examined chromosome aberrations in λ irradiated (3 Gy) lymphocytes from five patients with Alzheimer's disease (AD). In each case, the number of dicentrics was significantly higher than the number in irradiated lymphocytes from five age matched normal subjects, the mean value for AD cells being about 25% higher. There was no significant difference in number of acentrics between AD and normal cells. Examination of the number of first, second, and third division meta-phases, using fluorescence plus Giemsa staining, indicated that there was no difference in cycling time between AD and normal cells, and that after irradiation both groups showed the same mitotic delay. The similarity of our findings to those of others with irradiated Down's syndrome cells (from adult patients) is discussed.