Twenty original papers that reported on the effect of wheat bran on large bowel function were analysed. Bran increased the stool weight and decreased the transit time in each study in healthy controls and in patients with the irritable bowel syndrome, with diverticula, and with chronic constipation. Statistical evaluation of the data showed, however, that constipated patients had lower stool output and slower transit whether or not they had taken bran, and they responded less well to bran treatment than controls. From these data it is concluded that bran can be expected to be only partially effective in restoring normal stool weight and transit time in patients who are constipated.