1. Bowel transit time has been investigated in vegetarians and non-vegetarians and related to dietary fibre intake and the presence of diverticular disease. 2. Vegetarians who have less diverticular disease than non-vegetarians have more rapid transit times. 3. Subjects with total dietary fibre intake of more than 30 g/d all had transit times of less than 75 h whereas 38% of those eating less had transit times exceeding 75 h and varying up to 124 h. 4. Individuals with diverticular disease were found to have faster transit times than those without the disease. 5. The colon may respond to a fibre-depleted diet either by becoming hyperactive and prone to diverticular disease or by becoming hypoactive leading to constipation.