The influence of oxygen-derived free radical scavengers on survival in gastric cancer, with serosal invasion and metastases to the lymph nodes surrounding the stomach, was assessed in a prospective randomized controlled double-blind trial conducted for 5 years. To this end, allopurinol (inhibits the enzyme xanthine oxidase which is responsible for the formation of superoxide radicals and scavengers hydroxyl radicals) and dimethyl sulphoxide (DMSO; scavengers hydroxyl radicals) were used. Following potentially curative distal two-thirds partial gastrectomy, 228 patients making an uneventful recovery from surgery were randomized to the control group or to receive allopurinol (50 mg by mouth 4 times a day) or DMSO (500 mg by mouth 4 times a day). In 160 fully evaluable patients who were studied for 5 years, allopurinol and DMSO incurred a significant (p less than 0.01) survival advantage over the whole period of study. The similarity in efficacy between allopurinol and DMSO and the fact that the only action they share is scavenging oxyradicals suggest that these radicals mediate the aggressiveness of gastric cancer by producing tissue damage, thus allowing the cancer to spread. Consequently, oxygen-derived free radicals are implicated in the mechanism of gastric cancer, and removing them provides patients with a survival advantage.