OBJECTIVE : The role of oxidative stress in inflammatory bowel disease is increasingly recognised as an important factor. It is assumed that reduced levels of paraoxonase-1 (PON-1) and arylesterase (ARE) may lead to increased inflammation due to increased oxidative stress. This study aimed to investigate the relationship between ARE and PON-1 levels in ulcerative colitis (UC) patients and the difference in these levels in UC patients in comparison to the control group. METHODS : The study population consisted of 66 (73.3%) UC patients and 24 (26.7%) healthy individuals as the control group. The UC patients and the control group were compared in terms of PON-1 and ARE levels as oxidative stress markers. The UC patients were also grouped according to Mayo UC activity scores, and the differences in their PON-1 and ARE levels were assessed. RESULTS : The ARE values were statistically higher in the control group in comparison to the UC patients. Concentrations of PON-1 were not statistically different in the UC and control groups. The ARE value was found to be significantly lower in the UC patients with a haemoglobin level below 10 mg/dl. There was a correlation between the ARE and PON-1 values in the UC patients, but there was no difference between the ARE and PON-1 values, based on the UC patients' Mayo disease severity scores. CONCLUSIONS : This study found that the ARE values of UC patients were lower than those of healthy subjects. The same results could not be determined for PON-1. The data suggest that the antioxidative capacity of UC patients may be reduced.