Over a 4-year period in a chemoprevention trial on large bowel neoplasia, 58 patients with familial adenomatous polyposis were treated with 4 g of ascorbic acid (vitamin C)/day plus 400 mg of alpha-tocopherol (vitamin E)/day alone or with a grain fiber supplement (22.5 g/day). In this randomized, double-blind, placebo-controlled study, we determined the effects of these supplements on rectal polyps in these patients. Analysis by intent to treat suggested that the high-fiber supplement had a limited effect. Analysis adjusted for patient compliance showed a stronger benefit from the high-fiber supplement during the middle 2 years of the trial. The results provide evidence for inhibition of benign large bowel neoplasia by grain fiber supplements in excess of 11 g/day in this study population. The findings are consistent with the hypothesis that dietary grain fiber and total dietary fat act as competing variables in the genesis of large bowel neoplasia.