In a double-blind crossover study the effect of ingested guar gum granules, 15 g/d for 3 months, on blood lipids was evaluated in twelve obese hypercholesterolaemic patients. Serum total cholesterol decreased during the guar gum period in eight of the ten patients who completed the study. The mean serum cholesterol was 8.5 +/- 0.4 mmol/l after the guar gum period as compared to 8.9 +/- 0.3 mmol/l after the placebo period. The mean HDL-cholesterol or body weight showed no significant changes during either treatment period. Guar gum administration did not cause any significant changes in 24-h urinary excretion of sodium, potassium, calcium or magnesium. It is concluded that the administration of guar gum granules seems to reduce serum total cholesterol in hypercholesterolaemic patients without exerting an effect on HDL-cholesterol. The beneficial effect of guar gum is independent of the changes in body weight.