Prion diseases such as Creutzfeldt-Jakob disease (CJD) are associated in most cases with the accumulation of an unusual isoform of prion protein (PrPSC). PrPSC is derived from the abnormal folding of the cellular isoform of prion protein (PrPC). On the other hand, heat shock protein is known to ensure proper protein assembly and folding and to facilitate proteolytic digestion of abnormal or denatured proteins. Many studies have therefore hypothesized that heat shock protein is linked to prion disease. We examined the relationship between heat shock protein HSP70 and prion disease in CJD patients. HSP70 mRNA levels in mononuclear blood cells (MBCs) were compared in 14 CJD patients (10 confirmed by histo-pathological study), 12 vascular dementia (VD) patients, 16 patients with Parkinson's disease and dementia (PD) and 14 nondemented control subjects. The possible correlation between HSP70 mRNA expression levels and clinical findings was also evaluated. HSP70 mRNA expression levels in MBCs were measured by northern blotting. HSP70 mRNA levels in MBCs from patients with CJD were significantly higher than those from patients with VD or PD and in nondemented controls. Age at symptom onset, dementia severity, disease duration and neuroimaging grade of CJD patients were not correlated with relative HSP70 mRNA levels. No significant relationship between HSP70 mRNA levels and ageing was found. These results suggest that measurement of HSP70 mRNA in MBCs might provide an auxiliary tool for the diagnosis of CJD.