Guar gum, a storage polysaccharide galactomannan and a form of dietary fiber, was administered to 10 patients with type II a or b hyperlipidemia for 2 weeks. Five grams of gum was given before each of three meals daily, either in a specially prepared soup or mixed with fruit juice or milk. No other deliberate change of diet was made. Three patients had been taking 12 to 16 g/day of cholestyramine for more than 2 years and one had been taking 1000 mg of clofibrate daily. These drugs were continued throughout the trial. Serum cholesterol levels of all 10 patients had been stable for 6 to 18 months before the trial at the start of which the mean level was 345 +/- 15 mg/dl. After 2 weeks of guar gum the mean was 308 +/- 16 mg/dl, a fall of 10.6% (P less than 0.01). Serum triglyceride was not changed significantly. Guar gum, which can be incorporated into foods, merits further study as a potential hypocholesterolemic agent.