Low paraoxonase 1 (PON1) activity and carotid atherosclerosis have been suggested to be important risk factors for dementia. However, the studies to date could not fully clarify the relationship between PON1, carotid atherosclerosis and dementia. The present study aimed to measure carotid atherosclerosis and PON1 activity in Alzheimer's Disease and to evaluate the relationship between them. The study included 25 Alzheimer's patients and 25 control subjects, for a total of 50 individuals. The study measured the serum PON1 activity and other biochemical parameters and carotid atherosclerotic plaque values of the participants. The mean paraoxonase activity (31.06 +/- 2.31 U/L) was significantly lower in the Alzheimer's group compared to the control group (59.05 +/- 7.05 U/L) (P < 0.001). Nonetheless, the carotid plaque values were significantly higher in the patient group (3.02 +/- 0.52 mm) compared to the control group (1.84 +/- 0.45 mm) (P < 0.001). Furthermore, there was a negative correlation (81.0%) between PON1 activity and carotid plaque in the overall study group (P < 0.05). Also serum homocystein level was higher in the patient group (22.15 +/- 7.05) compared to the control group (13.30 +/- 3.32). In conclusion, our findings show inverse association between PON1 activity and carotid atherosclerosis in Alzheimer patients: the lower the PON1 activity the more progressed the atherosclerotic process in AD. (C) 2016 Polish Neurological Society. Published by Elsevier Sp. z o.o. All rights reserved.