The aim of the study. Investigation of the effect of vertical banded gastroplasty (VBG), which is an effective method of treating patients with morbid obesity on serum paraoxonase (PON) activity.Material and methods. Serum PON activity was measured in twenty eight morbidly obese patients 6 and 12 months after surgery. PON activity was also measured in the serum and liver of rats maintained on a restricted diet for one month.Results. We found that VBG-induced significant reduction in body weight and serum PON activity at 6 and 12 months after surgery. Similar patterns of decreases in serum paraoxonase activity in obese patients after VBG were observed in A, AB and B paraoxonase/esterase phenotypes. After VBG, several clinically relevant events occurred: a) a decrease of serum triacylglycerol concentration was observed; b) no significant changes in total serum cholesterol and LDL-cholesterol concentrations were found; c) serum HDL-cholesterol concentration increased slightly.Paraoxonase activity in the serum of rats maintained on a restricted diet, which induced approximately 30% and 50% of rat body weight and fat mass loss, respectively, was lower than in control animals.Conclusions. This study indicates that after VBG significant decreases in serum paraoxonase activity occur in obese subjects. It is likely that less food ingestion and possibly a different type of food consumed by the obese subjects after VBG (compared to type of food consumed before surgery) may contribute to decreases in serum PON activity.