We examined the effect of guar gum on glycemic control and serum lipid and lipoprotein profiles in mildly hypercholesterolemic patients with insulin-dependent diabetes. The study was done in a randomized, double-blind fashion with either guar gum or placebo added to the diet four times per day for 6 wk each. Fasting blood glucose and hemoglobin A1c decreased significantly during the guar-gum diet, whereas the diurnal glucose profile was unchanged. In addition, serum low-density-lipoprotein (LDL) cholesterol decreased by 20% and the ratio of LDL cholesterol to high-density-lipoprotein cholesterol by 28% during guar-gum therapy. No changes were seen in the placebo group. In conclusion, guar gum can improve glycemic control and decrease serum LDL-cholesterol concentrations in mildly hypercholesterolemic insulin-dependent diabetic patients and thus reduce risk factors for both micro- and macroangiopathic complications.