Bran, ispaghula (Fybogel), and lactulose were given to three groups of patients with diverticular disease for four weeks. Faecal weights, bile acids, fat and electrolytes, transit time, and colonic motility were estimated before and after treatment. Stool weight increased, notably with Fybogel. Cereal bran had the greatest effect on the transit time, reducing it significantly. There were no changes in faecal bile acids, fat or electrolytes. Coarse bran reduced colonic motility and the number of high pressure waves after food; Fybogel increased the basal pressure and was without effect on the food-stimulated pressures; whereas lactulose influenced neither. All agents paradoxically equally alleviated symptoms.