10 non-insulin-dependent (NIDDM) diabetic patients of the University Hospital Basle completed a placebo-controlled open crossover study of three 3-month periods in which the response to 30 g wheat bran and 15 g guar gum daily, respectively, was compared with placebo response. No significant difference occurred in blood glucose or HbA1c. Only postprandial plasma insulin levels were significantly lower in the afternoon at the end of both treatments (p less than 0.05). Serum total cholesterol levels decreased under guar gum by 10% (p less than 0.01), as did LDL cholesterol and the ratio of total to HDL cholesterol (p less than 0.05). Triglycerides were significantly lower (p less than 0.05) after 8 weeks. We conclude that neither guar gum nor wheat bran improves the metabolic control of NIDDM, but guar gum may be useful in patients with high serum total cholesterol levels.