To investigate the effect of urinary tract infection on oxidative stress in diabetic patients, we measured the activities of antioxidant enzymes such as catalase and superoxide dismutase, and lipid peroxidation levels in urine specimens of type II diabetic patents with urinary tract infection. A total of 69 patients were included into this study: 23 non-diabetic patients with urinary tract infection, 28 patients with diabetes mellitus, and 18 diabetic patients with urinary tract infection. Twenty-five healthy subjects, matched for age, sex, body mass index and smoking status were also included as control. Urine cultures were performed by the standard techniques, and all grown bacteria were identified as Escherichia coli. Antioxidant enzymes and lipid peroxidation levels in urine were measured by spectrophotometric method. In urine samples of diabetic patients with or without urinary tract infection and in urine samples of non-diabetic patients with urinary tract infection, catalase and superoxide dismutase activities were lower and lipid peroxidation levels were higher than those of the healthy subjects (p < 0.05). Diabetic patients without urinary tract infection were similar to non-diabetic patients with urinary tract infection. Decreased antioxidant capacity and the increased levels of lipid peroxidation were profoundly higher in diabetic patients with urinary tract infection. These results indicate that urinary tract infection aggravates the oxidative stress in diabetic patients. Therefore we believe that diabetic patients with urinary tract infection need antioxidant treatment.