The heat shock protein 70 (HSP70) gene is located in chromosome 14, it is now considered as a molecular 'chaperone' and a cell-protective agent. It may be closely related to the pathogenesis of dementia of the Alzheimer type (DAT). To examine the relationship between HSP70 and DAT, HSP70 mRNA expression levels in mononuclear blood cells (MBCs) from patients with DAT were measured by Northern blotting. We found no significant correlation between HSP70 mRNA levels and aging. We found that HSP70 mRNA levels in MBCs from patients with DAT were significantly lower than those from patients with vascular dementia and nondemented controls. These findings suggest that the lower levels of constitutive HSP70 mRNA in DAT play an important role in developing DAT and that the measurement of HSP70 mRNA may be useful for the diagnosis of DAT.