Human serum paraoxonase (PON1) is an HDL-associated enzyme involved in the protection of lipoproteins from oxidation. A polymorphism at position 192 (Gln/Arg) influences its activity in a substrate-dependent manner. The aim of the present study was to evaluate, in vivo, the contribution of the PON1-192 polymorphism to the protective effect of HDLs. Three hundred and forty seven subjects in the upper and lower decile of sex-specific HDL cholesterol distribution were selected from participants in a cardiovascular disease prevention study. PON1 genotypes were determined by PCR amplification and restriction analysis. Blood pressure, height, weight, smoking and alcohol habits, as well as the presence of familial or personal history of CHD were recorded. Plasma lipids and blood glucose were measured by routine enzymatic methods. As a measure of antiatherogenic HDL effect, carotid atherosclerosis was assessed by ultrasonography. Allele and genotype frequencies did not differ significantly between low- and high-HDL groups. Similarly, no significant difference was observed among genotypes in all variables studied. Subjects with Gln/Arg or Arg/Arg had more carotid abnormalities than Gln/Gln with an adjusted odds ratio (OR) of 3.27 (95% CI, 1.61-6.64, P=0.001) for abnormal carotid score. Stepwise logistic regression analysis showed that in the whole population age and presence of low-HDL were the only independent predictors for abnormal carotid score. In low-HDL, age was the only independent predictor entered into the model (OR 1.09/year, P<0.0001); in high-HDL, age entered first (OR, 1.07/year, P=0.001), followed by the presence of Gln/Arg or Arg/Arg (OR, 2.94, 95% CI, 1.47-5.91, P=0.002). In conclusion, in subjects with low levels of HDL cholesterol, carotid atherosclerosis is not related to PON1-192 polymorphism. On the other hand, in subjects with elevated concentration of HDL cholesterol, the presence of carotid atherosclerosis is significantly associated with the arginine variant in position 192 of the PON1 gene.