The influence of oxygen-derived free radicals on survival in advanced colonic cancer was assessed in a prospective randomized controlled double-blind trial using the radical scavengers dimethyl sulphoxide (DMSO) and allopurinol. Following palliative sigmoid colectomy for carcinoma at Dukes’ stage D, 306 patients were randomized to the control group or to electrocoagulation of liver secondaries alone or with allopurinol (50 mg by mouth 4 times a day) or DMSO (500 mg by mouth 4 times a day). In 193 fully evaluable patients who were studied for 5 years, allopurinol and DMSO incurred a significant (p < 0.05) 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 detrimental effects of malignancy and that removing them incurs a survival advantage for patients with advanced colonic cance