Carbohydrate absorption was assessed during acarbose administration to investigate the actions of this drug. In 7 healthy volunteers, breath hydrogen concentration was measured at 15-min intervals after administration of 6 g of lactulose, and continued until 4 h after the breath hydrogen level exceeded its pretreatment value by Ն10 ppm, then the amount of undigested carbohydrate was calculated following administration of various doses of acarbose and Ensure Liquid. Breath hydrogen data were also obtained before and after administration of acarbose to 8 patients with Type 2 diabetes mellitus for 2 and 4 months. After administration of 50 mg of acarbose with 250 ml or 500 ml of Ensure, the mean amount of unabsorbed carbohydrate was 5.3 g and 7.7 g, respectively, while unabsorbed carbohydrate increased to 10.8 g after 100 mg of acarbose with 500 ml of Ensure. In the diabetic patients, breath hydrogen excretion decreased to 31.6 % of baseline after 2 months of acarbose administration, indicating decreased carbohydrate malabsorption. Despite this, the haemoglobin A 1c level remained stable after 5 months. In conclusion, the extent of carbohydrate malabsorption depended on the acarbose dose and the carbohydrate load. Although carbohydrate malabsorption decreased with continued acarbose administration, the improvement of glycaemic control was maintained.