Previous studies have suggested that Helicobacter pylori (H. pylori) infection may play an important role in the process of atherosclerosis. The objective of this study was to investigate serum paraoxonase and arylesterase activities, and lipid hydroperoxide (LOOH) and total thiol (SH) levels along with lipid parameters in H. pylori infected subjects. Fifty-six H. pylori positive subjects and 43 H. pylori negative subjects were enrolled. Serum paraoxonase and arylesterase activities were measured spectrophotometrically. LOOH levels were measured by FOX-2 assay. Serum SH levels, paraoxonase and arylesterase activities were significantly lower in H. pylori positive group than H. pylori negative group (all p<0.05), while LOOH levels were significantly higher (p<0.05). In H. pylori positive subjects, serum LOOH levels were correlated with SH levels (r=-0.247, p<0.05), serum paraoxonase (r=-0.432, p<0.05) and arylesterase activities (r=-0.404, p<0.001), and triglyceride (r=0.305, p<0.05), total cholesterol (r=0.568, p<0.05), high-density lipoprotein-cholesterol (HDL-C) (r=-0.300, p<0.05) and low-density lipoprotein-cholesterol (LDL-C) (r=0.577, p<0.05) levels. Serum paraoxonase and arylesterase activities were also correlated with HDL-C levels (r=0.221, p<0.05 and r=0.291, p<0.05, respectively), while no correlation was observed with triglyceride, total cholesterol and LDL-C levels (both p>0.05). In conclusion, paraoxonase and arylesterase activities decrease significantly in H. pylori infected subjects. Lower serum paraoxonase-1 (PON1) activity seems to be related to decrease in HDL-C and, in part, to increased oxidative stress and inflammatory condition induced by H. pylori infection. Measurement of serum PON1 activity may help in the early identification of H. pylori infected subjects with increased risk of atherosclerotic disease.