Acarbose (Bay g 5421) is a powerful α-glucoside hydrolase inhibitor of potential value in the treatment of diabetes and hypoglycemic dumping syndrome after gastric surgery. The extent of its use may be limited by symptoms produced by carbohydrate malabsorption. To minimize these, the action of low doses of acarbose on 24-h blood glucose profiles and hydrogen evolution have been studied on four ambulant volunteers on control diets, after exclusion of sucrose and also after addition of guar in an attempt to enhance the therapeutic effect. Replacement of dietary sucrose by starch abolished significant hydrogen evolution in the morning after low doses of acarbose but did not reduce its effectiveness in decreasing the mean three-meal blood glucose area by 41% (P < 0.002). Addition of hydrated guar to this diet reduced the mean three-meal glucose area after acarbose further by 72% (P < 0.001) but increased hydrogen evolution. The results suggest that acarbose will be both effective and acceptable given at low dose when the dietary carbohydrate is starch.