Guar gum is a dietary fiber reported to decrease plasma cholesterol concentration. This study investigated the effect of guar therapy in 50 men with moderately elevated plasma cholesterol who were randomized to an 8-week study of guar therapy. Three forms of guar gum were used: a medium viscosity solid or liquid form, a high viscosity liquid form or placebo. When the medium viscosity guar therapy groups were combined, 4 weeks of therapy were shown to result in a substantial reduction in total and low density lipoprotein (LDL) cholesterol of 25 mg/dl and 23 mg/dl (p = 0.035 and 0.12), respectively. The high viscosity guar group had a reduction in total cholesterol and LDL cholesterol of 37 and 30 mg/dl, respectively (p less than 0.003 and p less than 0.02). Following 8 weeks of therapy, a return toward baseline values was observed. No significant changes were demonstrated in blood chemistries, triglyceride values, total high density lipoprotein (HDL) cholesterol or the HDL2 fraction of HDL cholesterol. The effect of the solid and liquid forms of guar on plasma cholesterol reduction was similar. This study shows that a nonpharmacologic dietary additive reduces plasma total and LDL cholesterol.