Reactive oxygen species have been implicated in Helicobacter pylori-mediated gastric carcinogenesis, whereas diets high in antioxidant vitamins C and E are protective. We have examined the effect of vitamin C and E supplements in combination with H. pylori eradication on reactive oxygen species activity in H. pylori gastritis. H. pylori-positive patients were randomized into four groups: triple therapy alone (Bismuth chelate, tetracycline, and metronidazole for 2 weeks), vitamins alone (200 mg vitamin C and 50 mg vitamin E, both twice per day for 4 weeks), both treatments or neither. Plasma and mucosal ascorbic acid, malondialdehyde and reactive oxygen species were determined before and after treatment. Compared with normal controls (n 61), H. pylori-positive patients (n 117) had higher mucosal reactive oxygen species and malondialdehyde levels and lower plasma ascorbic acid. Plasma ascorbic acid doubled in both groups of patients receiving vitamins and mucosal levels also increased. Malondialdehyde and reactive oxygen species fell in patients in whom H. pylori was eradicated but vitamin supplements were not effective either alone or in combination with H. pylori eradication. Supplements of vitamins C and E do not significantly reduce mucosal reactive oxygen species damage in H. pylori gastritis.