Paraoxonase 1 (PON1) is an HDL-associated enzyme which protects HDL and LDL particles from lipid peroxidation. Its enzymatic serum activity varies 10-40-fold between individuals, and its biallelic gene polymorphism at codon 192 (glutamine-->arginine, Gln/Arg) has been associated with coronary artery disease in diabetic patients. To evaluate the role of this PON1 gene polymorphism in cerebrovascular disease, we determined the PON1 192 genotype in 149 patients with hemodynamically relevant extracranial artery stenosis and in 241 controls. The PON1 192 Gln/Arg genotype was determined using polymerase chain reaction followed by Alw I digestion and polyacrylamide gel electrophoresis. Among all subjects, there was no association between the PON1 192 Gln/Arg genotype and cerebrovascular disease (Odds ratio for Arg/Arg and Gln/Arg vs Gln/Gln 0.99, 95%-CI 0.70-1.39). In contrast, in the subgroup of type 2 diabetic patients the PON1 192 Arg allele conferred about twice the risk of cerebrovascular stenosis compared to those homozygous for the Gln allele (Odds ratio 2.00, 95%-CI 0.92-4.38). Our data indicate that in the general population the PON1 192 Gln/Arg gene polymorphism cannot be regarded as a major risk marker for cerebrovascular disease. The observed interaction with type 2 diabetes, however, is supporting the hypothesis that the effect of the PON1 192 Arg allele on atherosclerosis is modulated by other risk factors like diabetes.