N-truncated and N-modified forms of amyloid-β (Aβ) peptide are found in diffused and dense core plaques in Alzheimer's disease (AD) brain. Among them, the most abundant N-truncated peptide starts with pyroglutamyl at residue 3 (AβpE3). AβpE3 has increased aggregation potential and toxicity and its abundance has been reported to correlate with the severity of the clinical phenotype in AD patients. N-terminal glutamate conversion generating AβpE3 is catalyzed by glutaminyl cyclase. This enzyme was found to be upregulated in the cortex of patients with AD. In the present study, we investigated glutaminyl cyclase mRNA and protein expression in peripheral blood from AD patients and age-matched controls. Higher levels of glutaminyl cyclase mRNA and protein were present in AD patients compared with controls. Interestingly, we observed a correlation between glutaminyl cyclase expression and the severity of dementia (value of Mini-Mental State Examination). Therefore, we propose glutaminyl cyclase dosage in peripheral blood as a potential biomarker of AD.