The metabolic effects of an increased dietary content of cereal fibre were studied in 14 Type 2 diabetic subjects. They were given two isoenergetic diets in randomised order during two consecutive 3-week periods. A diabetic diet, containing 18.9 g dietary fibre/6.7 MJ (1600 kcal), was compared with a diet of identical composition except for an increased content of cereal fibre (42.4 g dietary fibre/6.7 MJ). The mean blood glucose level and the urinary glucose excretion were significantly lower in patients on the cereal-fibre-rich diet, while the serum insulin concentrations were similar. The mean blood glucose level was significantly reduced at 0700 h by 6% (p less than 0.05) and at 1100 h by 13% (p less than 0.01) on the high-fibre diet. Consequently the insulin/glucose ratio was higher (33%, p less than 0.02) in patients on the fibre-enriched diet. There were only minor differences with regard to the serum lipoprotein concentrations. The lipoprotein lipase activities were similar in the two dietary groups. The reduction of blood glucose concentrations together with unchanged serum insulin concentrations is compatible with improved peripheral insulin sensitivity.