The Irritable Bowel Syndrome (IBS) is one of the most common conditions referred to a gastroenterologist, but there is no consensus of opinion regarding either its aetiology or its treatment. This is partly due to an absence of any physiological marker, and to the variability of presenting symptoms. It has been suggested that it is due to a fibre depleted diet, although this has never been shown. The diets of 187 patients with the IBS and 61 control subjects were investigated using a validated dietary diary method. The patients were subdivided according to presenting complaint. It was found that patients had a lower intake of dietary fibre than controls,and in particular those with diarrhoea had a low intake of fruit fibre. Patients with constipation had a lower food intake. To help in the dietary assessment of patients a computer programme based on a simple prospective dietary questionnaire was written. It proved to be an effective and rapid guide to fibre intakes. Gut transit times were measured using the breath hydrogen technique for mouth to caecum transit times and the continuous marker technique for whole gut transit times. The possible effect of abnormal transit through the gut on food handling was investigated. It was found that patients presenting with diarrhoea had significantly shorter mouth to caecum transit times (with implications for the absorption of food) and that patients with constipation had lengthened transit times, both mouth to caecum and whole gut. Malabsorption of simple sugar solutions leading to gut symptoms was demonstrated both in patients and controls. Finally, 11 patients with food intolerance were investigated, as it has been suggested that food intolerance is a major factor in the aetiology of the IBS. The results suggest that low fibre intakes are probably important in the aetiology of IBS and that, at least in some individuals, carbohydrate malabsorption may also be a significant factor.