In the present study a randomized cross-over design was used to determine the clinical usefulness of adding 16 g of beet fiber to the ordinary diet of non-insulin dependent diabetic (NIDDM) out-patients. In addition, fiber effects on the gastrointestinal hormone responses to a standardized test meal were evaluated. The study included five patients treated with diet alone and eight patients treated with diet and sulphonylurea (SU). Beet fiber supplementation resulted in a 10% reduction (P less than 0.01) of serum cholesterol in SU-treated patients. No differences were found for fasting blood glucose, glycated hemoglobin, serum triglycerides or body weight. In the diet-treated patients, fasting plasma somatostatin was elevated during the fiber period. However, postprandial responses of insulin, C-peptide, glucagon, gastric inhibitory peptide and somatostatin were not influenced by an increased fiber intake in any group. All patients experienced mild gastrointestinal discomfort during the fiber period. In view of the limited metabolic benefit of beet fiber treatment we conclude that there is little use for this type of dietary fiber in the routine treatment of patients with NIDDM.