OBJECTIVE Reactive oxygen species are regarded as mediators of inflammation in ulcerative colitis. Antioxidant enzymes are the primary defense against reactive oxygen species. We investigated the activities of antioxidant enzymes-superoxide dismutase, catalase and glutathion peroxidase-in colonic mucosa biopsy specimens obtained at different phases of the disease in patients given vitamin E therapy or not and in controls. METHODS Biopsy specimens were obtained from 29 patients with ulcerative colitis and from 18 healthy controls. Ten patients on maintenance drug therapy were also given a 300 mg daily oral dose of vitamin E. RESULTS Enzyme activities were significantly increased in inflamed non-polypous colonic mucosa in patients with active ulcerative colitis, while they were similar to activities observed in controls during quiescent phases of the disease. Antioxidant enzyme activities in specimens from inflammatory polyps remained high even in patients in clinical remission. Administration of vitamin E resulted in a significant reduction of glutathion peroxidase activity in the colonic mucosa. CONCLUSIONS These findings indirectly confirm that important quantities of reactive oxygen species are formed in ulcerative colitis. Further studies are needed to elucidate the relationship of these changes with the pathogenesis of ulcerative colitis.