High-fibre diets, particularly with the addition of guar gum, have been reported to improve control of blood glucose concentration in diabetics. These studies are reviewed, and the results of a study of 22 obese, poorly controlled, poorly compliant diabetic outpatients are presented. In a random, single-blind controlled trial, either guar, or bran, or placebo were added to the previous diet. Over a three-month period, there were no changes in weight, fasting blood glucose levels, or random blood glucose levels. Problems of tolerance were experienced with guar gum. Therefore, supplementation of the diet of such patients with guar gum or bran does not produce long-term improvements in diabetic control in a clinical context. The use of naturally occurring high-fibre foods may slightly improve diabetic control in motivated patients, but further long-term trials are required to establish whether dietary fibre will have any significant role in the practical management of diabetes.