Glucomannan (Propol), a potent gel forming dietary fibre, was added to a carbohydrate rich breakfast in eight patients with previous gastric surgery suffering from postprandial hypoglycaemia. Addition of only 2.6 g and 5.2 g glucomannan to the meal dose dependently improved reactive hypoglycaemia from 2.3 (0.2) mmol/l to 3.3 (0.2) mmol/l (p less than 0.0005) after 2.6 g and 4.1 (0.2) mmol/l (p = 0.0005) after 5.2 g, and decreased postprandial rise in plasma insulin (p less than 0.05). Expiratory breath hydrogen excretion tended to decrease reflecting improvement of carbohydrate metabolism. Addition of glucomannan to an intraduodenal sucrose solution significantly raised plasma glucose nadirs, indicating glucomannan to be effective during the intestinal phase. It is concluded that small amounts of glucomannan may be beneficial to patients with reactive postprandial hypoglycaemia, without the disadvantage of unpalatability and carbohydrate malabsorption.