A study has been made of aneuploidy in the cultured lymphocytes of senile dements (46 females and 8 males), arteriosclerotic dements (10 females and 8 males) and normal geriatric controls (55 females and 18 males). Contrary to previous reports of senile dements having a higher degree of hypodiploidy (chromosome loss) than age-matched controls, there were no significant differences between any of the diagnostic groups. The association between individual subjects' age and hypodiploidy is positive, though not significant, with a high degree of individual variation. The reasons for the discrepancies between this and previous studies are discussed as are possible future lines of research in this field.