Thirty-nine patients with noninsulin-dependent diabetes on oral drug treatment were randomly allocated to either guar gum or placebo treatments for 3 mo. After 3 mo the placebo group was switched to guar gum treatment and both groups were followed for 10 mo (open trial). No significant difference occurred in the fasting blood glucose or glycosylated hemoglobin A1 levels between the two groups at 3 mo. Serum total cholesterol level decreased in the guar gum group from 6.55 +/- 1.45 to 5.69 +/- 1.2 mmol/L (p less than 0.001) but no changes were observed in the placebo group (6.55 +/- 1.2 vs 6.26 +/- 1.4 mmol/L, NS) during 3 mo. At the end of the open trial (n = 33), serum cholesterol was still approximately 7% lower than before guar gum treatment. No consistent changes occurred in serum HDL-cholesterol or triglycerides. Serum vitamin A level was slightly lowered and plasma zinc level elevated during the open trial. Serum vitamin E level was decreased only in the group switched to guar gum at 3 mo.