Abstract Serum paraoxonase-1 (PON1) binds mainly to high density lipoproteins (HDLs) and protects low density lipoproteins (LDLs) against oxidation. While paraoxonase and arylesterase activities are traditionally assayed, lactonase activity, accounting for protection against LDL oxidation, was less investigated in obese children and adolescents. Therefore, we aimed to measure lactonase, paraoxonase and arylesterase activities, oxidized LDL (ox-LDL) and malondialdehyde (MDA) levels in obese children and adolescents. Study population included 68 children (35 obese and 33 normal-weight). Arylesterase and paraoxonase activities were assayed spectrophotometrically. Lactonase activity, ox-LDL and MDA levels were measured using a pH-sensitive colorimetric assay, an ELISA technique and a fluorimetric method, respectively. The lipid profile was assessed by common methods. Lactonase and arylesterase activities were decreased in the presence of obesity. MDA, but not ox-LDL levels, showed significant differences between groups. Multiple regression analysis identified a reciprocal relationship and a possible association between lactonase and arylesterase activities and obesity.