OBJECTIVE Psoriasis is a chronic immune-mediated inflammatory skin disease associated with metabolic syndrome, which is made up of a cluster of disorders, including obesity, diabetes mellitus, dyslipidemia, and cardiovascular disease. The aim of this study was to investigate serum paraoxonase-1 activities and oxidative status parameters in patients with plaque-type psoriasis with or without metabolic syndrome. METHODS In this study, patients with plaque-type psoriasis with (n=25) or without (n=27) metabolic syndrome, according to the criteria of the International Diabetes Federation (IDF), were matched for age and sex to an equally sized control group (n=25). RESULTS In patients without metabolic syndrome, serum paraoxonase and arylesterase activities showed mean decreases of 29 and 6%, respectively, whereas in patients with metabolic syndrome, the mean decreases in the enzymes' activities were 35 and 11%, respectively, compared with those in the control group. Serum total antioxidant capacity and total oxidant status were not statistically significant in any of the three groups. Multiple linear regression analysis revealed that HDL cholesterol and log-transformed triglyceride were independent variables for serum arylesterase activity and that fasting glucose and diastolic blood pressure were independent variables for serum paraoxonase activity. CONCLUSIONS These results suggest that according to the criteria of the IDF, the significant decrease observed in serum paraoxonase activity was independent of the metabolic syndrome in patients with mild-to-moderate plaque-type psoriasis, whereas the significant decrease in serum arylesterase activity was associated with the metabolic syndrome.