Whole-grain foods such as oats may protect against colorectal cancer and have benefits on inflammatory bowel disease and coeliac disease. The present study aimed to systematically review the literature describing intervention studies that investigated the effects of oats or oat bran on risk factors for bowel disease. A literature search was conducted using Embase, Medline and the Cochrane library, which identified 654 potential articles. Thirty-eight articles describing twenty-nine studies met the inclusion criteria. Two studies carried out in participants with a history of colorectal adenomas found no effects of increased oat-bran intake on indirect risk makers for colorectal cancer. One of two interventions with oat bran in patients with ulcerative colitis showed small improvements in the patients' conditions. Most of the eleven studies carried out in adults with coeliac disease showed no negative effects of uncontaminated oat consumption. The fourteen studies carried out in volunteers with no history of bowel disease suggest that oats or oat bran can significantly increase stool weight and decrease constipation, but there is a lack of evidence to support a specific effect of oats on bowel function compared with other cereals. A long-term dietary intake of oats or oat bran could benefit inflammatory bowel disorders, but this remains to be proven. A protective effect on colorectal adenoma and cancer incidence has not yet been convincingly shown. The majority of patients with coeliac disease could consume up to 100 g/d of uncontaminated oats, which would increase the acceptability of, and adherence to, a gluten-free diet.