Nineteen patients with primary hypercholesterolaemia previously stabilized on diet alone were treated with a new formulation of guar gum (6g t.d.s. with meals) in a placebo-controlled, single-blind study. Seventeen patients completed 3 months treatment without serious side effects, while 2 patients withdrew immediately because of severe diarrhoea. Thirteen patients have completed 12 months treatment with guar gum. There have been no significant changes in safety parameters. Plasma cholesterol was reduced by a significant 15% during the first 3 months of treatment (7.9 +/- 0.8 vs 6.7 +/- 1.0 mmol/l, P less than 0.001) and this effect has been sustained for 12 months. The fall in plasma cholesterol was associated with a significant 20% fall in LDL cholesterol, but with no change in HDL cholesterol. Plasma triglycerides did not change significantly. Percentage cholesterol absorption was reduced by guar gum in 4/5 normal subjects examined.