Serum paraoxonase-1 (PON1) is a high-density lipoprotein-associated enzyme that can inhibit low-density lipoprotein (LDL) oxidation in vitro. The role of PON1 in vivo still remains to be clarified. We investigated the effect of PON1 genotype (-107C > T and 192Q > R), concentration, paraoxonase activity, and arylesterase activity on the early phase of lipid peroxidation in plasma samples of 110 patients with heterozygous familial hypercholesterolemia. The degree of lipid oxidation was assessed by quantitation of oxidized-linoleic acid (the most abundant fatty acid present in LDL) using high performance liquid chromatography. We found a significant inverse correlation between paraoxonase activity and the oxidized-linoleic acid concentration (r = -0.22, P = 0.03), independent of baseline linoleic acid levels. These findings support an anti-oxidative role for PON1 in patients with FH, and thus may give insight into the functioning of PON1 in vivo.