A Cochrane review included 20 randomised trials with a total of 211,818 subjects, assessing beta-carotene (12 trials), vitamin A (4 trials), vitamin C (8 trials), vitamin E (10 trials), and selenium (9 trials). Trial quality was generally high. Meta-analysis (random effects; RR 0.94, 95% CI 0.83–1.06) showed no significant effects of supplementation with antioxidants on the incidences of gastrointestinal cancers. The fixed effect model meta-analysis showed that antioxidant supplements significantly increased mortality (RR 1.04, 95% CI 1.02–1.07). Beta-carotene and vitamin A (RR 1.16, 95% CI 1.09–1.23) and vitamin E (RR 1.06, 95% CI 1.02–1.07) significantly increased mortality. In five trials (four with high risk of bias), selenium seemed to show significant beneficial effect on gastrointestinal cancer occurrence (RR 0.59, 95% CI 0.46 to 0.75. The potential cancer preventive effect of selenium should be studied in adequately conducted randomised trials.