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The aggregation of amyloid-β_{42} (Aβ_{42}) constitutes one of the major pathogenic events in Alzheimer's disease (AD), and the study of regional cerebral blood flow (rCBF), using single photon emission computed tomography (SPECT), aids the diagnosis of AD. In this study, we evaluated whether there was a correlation between rCBF in brain regions and plasma levels of Aβ_{1-42} in AD. 29 patients (mean age 71 ± 9) with a diagnosis of AD who fulfilled NINCDS-ADRDA criteria with a mean Mini-Mental Status Examination score of 15 ± 9 and 16 normal controls (mean age 64 ± 8) underwent SPECT brain imaging with hexamethylpropylene amine oxime, and semiquantitative analysis of rCBF was performed. Plasma samples were collected the same day of the SPECT and plasma Aβ_{1-42} measured by ELISA. A significant reduction of rCBF was observed in most regions in AD compared to controls, whereas mean plasma Aβ_{42} did not differ between the two groups. There was no correlation between rCBF in any region and plasma Aβ_{42} nor any correlations between gender, age, and severity with plasma levels of Aβ_{42}. Since rCBF is coupled to neuronal activity, we conclude that plasma Aβ_{1-42} concentration is independent of neuronal function in every single region of the brain. [ABSTRACT FROM AUTHOR]