The aggregation of amyloid-beta42 (Abeta42) 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 Abeta1-42 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 Abeta1-42 measured by ELISA. A significant reduction of rCBF was observed in most regions in AD compared to controls, whereas mean plasma Abeta42 did not differ between the two groups. There was no correlation between rCBF in any region and plasma Abeta42 nor any correlations between gender, age, and severity with plasma levels of Abeta42. Since rCBF is coupled to neuronal activity, we conclude that plasma Abeta1-42 concentration is independent of neuronal function in every single region of the brain.