The short-term metabolic effects of a dietary supplementation of biscuits containing raw fibre and lactulose (Fiberlac, Bracco) on circadian glucose, insulin and amino acid concentrations were studied in 10 obese patients in a crossover comparison. The biscuits (3 at breakfast, 4 at lunch and 4 at evening meal; approximately 10 g total dietary fibre, 2 g raw fibre, 8.25 g lactulose) were randomly substituted for an equicaloric part of a diet containing 20-22 kcal per kg ideal body weight under strict medical surveillance. Blood glucose in response to meal, as well as mean concentration throughout the day was lower during fibre supplementation. Also mean insulin was halved, and the insulin response to meals was blunted by 100-250 pmol/L. The plasma amino acid response to meals was increased, possibly in relation to decreased insulinemia. The data show a remarkable metabolic effect of the preparation in obese patients, without any further dietary restriction. The clinical effects and compliance remain to be determined in long-term studies, and in other states of glucose-intolerance, e.g. liver cirrhosis.