The effect of guar (15.6 g/day), a dietary fibre, and simultaneous administration of bezafibrate (600 mg/day) during dietetic treatment on the plasma lipoproteins and apolipoproteins was investigated in 12 patients with familial hypercholesterolemia (corresponding to the HLP type IIa pattern). Either bezafibrate alone or bezafibrate in combination with guar was administered in a cross-over study for 3 months. Guar led to an additional lowering of the total cholesterol in the plasma by 7% (P less than or equal to 0.05) associated with a fall of the low density lipoprotein cholesterol (LDL-cholesterol (LDL-cholesterol) by 13% (P less than or equal to 0.01) without any changes in the very low density lipoprotein (VLDL) and high density lipoprotein (HDL) cholesterols. In parallel with the decrease in LDL-cholesterol, the apoprotein B also was diminished by 20% (P less than or equal to 0.05). The plasma triglyceride level and the triglyceride distribution within the individual lipoprotein fractions were not altered in any consistent manner by the addition of guar. Neither the fasting plasma glucose level nor the body weight were affected. The side-effects due to guar treatment consisted of slight nausea, meteorism and constipation, but this did not in any of the cases lead to early termination of the study. These results demonstrate that guar exerts its cholesterol-lowering effect in addition to that of bezafibrate.