The paraoxonase (PON1) Gln192-->Arg polymorphism was examined in a group of sporadic late-onset Alzheimer's disease (AD) patients, in a group of coronary artery disease (CAD) patients, and in normal subjects. The AD sample showed a PON1*R allele frequency significantly lower than the control group (0.225 vs. 0.281, P=0.049). In the CAD patients the *R allele was more frequent than in the controls (0.230 vs. 0.213), though not significantly (P=0.28). The odds ratios (OR) adjusted for age, gender, and APOE polymorphism by logistic regression analysis highlighted that in AD the PON1 RR genotype was significantly protective (OR=0.41, 95% CI=0.19-0.90; P=0.025), whereas in CAD it appeared to be a significant risk factor (OR=5.11, 95% CI=1.09-23.9; P=0.038) limited to younger patients.