This study was designed to evaluate the oxidative and antioxidative status in patients with ulcerative colitis by detecting antioxidant enzyme paraoxonase I activity together with the level of a well-known marker of oxidative stress, malondialdehvde. Serum paraoxonase 1 activity and malondialdehyde levels were analysed in 30 patients with ulcerative colitis and 30 controls using a spectrophotometric method; correlation analysis was made between these variables. Serum malondialdehyde levels were higher in the ulcerative colitis group (median: 2.5, range: 0.5-9.4 nmol ml(-1)) than among the controls (median:1.1. range: 0.5-2.3 nmol/ml(-1); p < 0.001) whereas paraoxonase I activities were lower in the ulcerative colitis group (median: 158.4, range: 61.6 -264.1Ul(-1)) than in the control group (median: 233.3, range: 114.4-431.0 Ul(-1); p < 0.001). There was no correlation between serum malondialdehyde level, paraoxonase 1 activity and disease activity. (1) Increased reactive oxygen metabolites levels in ulcerative colitis may result in a pro-oxidation environment, which in turn could result in decreased antioxidant paraoxonase 1 activity and increased malondialdehyde levels, (2) increased cytokines may be a possible cause of decreased paraoxonase 1 activity and (3) decreased serum paraoxonase 1 activity may be a part of an inflammatory response. Copyright (c) 2005 John Wiley & Sons, Ltd.