Epidemiological studies have emphasized the importance of diet in the etiology of large bowel cancer. A high intake of dietary fibre has been hypothesized to decrease the risk of colorectal cancer. This hypothesis is now being tested within the framework of intervention studies. Supplemental fibre is being used in four chemopreventive trials as wheat bran (22.5 g/day together with vitamins C and E, New York study; 25 g/day, Australian study; 50 g/day, Toronto study; 13.5 g/day, Arizona study) and in one trial as ispaghula husk (a mucilaginous substance) at 3.8 g/day (European Cancer Prevention Organization study). One study is evaluating the effect of fibre on adenoma size in patients with familial polyposis and an intact rectum, and the other four studies are focusing on the recurrence of adenoma in patients with previous adenomas. The results available so far provide some evidence for the inhibition of adenoma growth through a high-fibre and/or low-fat diet. Further results are expected within a year. If there is sufficient evidence for a protective effect of dietary fibre on colorectal carcino-genesis, a simple, safe and inexpensive prophylaxis for a very common cancer will be indicated.