Dietary supplementation with wheat bran has been widely advocated as a first-line treatment of patients with the irritable bowel syndrome (IBS). Few controlled trials have been reported, and the results are, furthermore, contradictory. The present study comprised 20 patients with IBS, of whom 18 (14 women, 4 men) completed the trial. The two treatment periods of 6 weeks each, with a daily intake of 30 g coarse wheat bran or 30 g placebo bran, respectively, were randomized in a double-blind cross-over design. Wheat bran significantly (P less than 0.05) increased the stool weight and shortened the intestinal transit time but was without significant effect on the colonic motility index was shown. We conclude that coarse wheat bran used as the only treatment in IBS does not provide a sufficient effect in a 6-week period. However, wheat bran seems to be justified in the treatment of constipation.