We questioned whether a dietary fibre supplement known to lower fasting plasma cholesterol concentrations can also lower the postprandial plasma cholesterol, glucose and insulin concentrations when it is administered just before a meal. Two studies were conducted in healthy middle-aged volunteers of both sexes in whom the fasting plasma total cholesterol concentrations were above normal. In the first study the dietary fibre treatments (2.2 g) were psyllium and a psyllium-citrus pectin mixture to which the subjects (four males, eight females) had no prior exposure. Controls received no supplement. The meals were high-fat breakfasts and lunches. In the second study the dietary fibre (6 g) was from sugar-beet root and the reference control was alpha-cellulose (2 g); the meal was of glucose. The volunteers (eight males, eight females) had prior exposure to the fibre supplements three times daily for 3 weeks. After adjustments for fasting values and changes in haemodilution, the psyllium and psyllium-citrus pectin mixture in the first experiment had no significant effects on the postprandial measurements of plasma glucose, insulin:glucose ratio, total-, LDL- and HDL-cholesterol, and triacylglycerol. By contrast, the sugar-beet fibre in the second study significantly decreased the area under the glucose response curve by 6.9%, the area under the insulin response curve was lower by 9.6%, although not significantly, and the post-glucose meal HDL-cholesterol concentration was significantly (12%) higher. Additionally, the 3-week treatment with sugar-beet fibre significantly lowered the fasting total- and LDL-cholesterol concentrations, by 8.5% and 9.6% respectively. We conclude that low doses of psyllium and citrus pectin at breakfast and lunch have no effects on the postprandial plasma measurements, but that sugar-beet fibre taken daily for 3 weeks affects both fasting and postprandial plasma metabolites favourably in these individuals with mildly increased risk of ischaemic heart disease. Further, we observed that small changes in haemodilution occur after meals, as indicated by plasma albumin concentration and packed cell volume. Underemphasis of the dietary fibre effects may occur when postprandial haemodilution is not taken into account.